Posts Tagged ‘ analgesic ’

Opioid Analgesics (Morphine) & Equine Colic (Butorphanol)


The primary effect of opioids is to temporarily remove pain when used at therapeutic levels; this is done by binding to opioid receptors found primarily in the central nervous system (some receptors are found in the gastrointestinal tract). When larger doses are given, opioids can induce beneficial and non-beneficial pharmacological effects such as sedation, respiratory depression or constipation. The term given to non-synthetic opioids is opiates; opiates are derived from the naturally occurring opium alkaloids found in the resin of the opium poppy.

The main uses of opioids include:

  • Treatment of acute pain (e.g. post-operative)
  • Palliative care to alleviate serve chronic pain (e.g. cancer)
  • Surgical premedication regimes (due to their calming, sedative action – also reduces the amount of post pain relief required)
  • Neuroleptanalgesia (a state of quiescence, altered awareness, and analgesia produced by a combination of an opioid analgesic and a neuroleptic – a tranquilliser). And neuroleptanaesthesia (a form of anaesthesia achieved by the administration of a neuroleptic agent, a narcotic analgesic, and nitrous oxide with oxygen. Induction of anaesthesia is slow, but consciousness returns quickly after the inhalation of nitrous oxide is stopped)
  • Restraint
  • Antitussive (the alleviating or suppressing coughing)


The body naturally releases endogenous opioid peptides or endorphins which bind to opioid receptors in the body. There are three primary receptor types, each with different functional responses and these are:

  • μ (mu) – Responsible for supraspinal (above the spine) analgesia, respiratory depression, euphoria and physical dependence of opioids (misuse and abuse of opioids)
  • κ (kappa) – Responsible for spinal analgesia, miosis (pupil constriction of the eye) and sedation
  • δ (delta) – Responsible for hallucinations and dysphoria (agitation and anxiety)

Exogenous opioids (synthetic or natural) mimic the body’s own endogenous opioids and are therefore able to bind to the above receptors – resulting in a response specific to the receptor they bound.  Opioids are able to either stimulate or depress the receptors, meaning opioid drugs can be classes as; agonists, antagonists or both – agonists-antagonists. Agonists bind to receptors and induce pharmacological responses whereas antagonists bind to receptors and do not produce a response, this makes them able to counteract the effect of other drugs or endogenous compounds.

Agonists are used for the primary reasons listed earlier, mainly analgesia. Examples of opioid agonists are:

  • Morphine
  • Pethidine
  • Methadone
  • Fentanyl
  • Etorphine.

Antagonists are primarily used to reverse the effects of agonists i.e. analgesia. They do this by binding to μ and κ opioid receptors, which together are responsible for analgesia. Examples of opioid antagonists include:

  • Naloxone (Narcan)

Agonists-Antagonists have both agonistic and antagonistic properties. This means they are able to antagonise the pure agonists (e.g. morphine) at μ and κ opioid receptors but they also have their own milder agonistic effects. The agonist effect is sufficient enough to be used as analgesics. Examples of opioid agonists-antagonists include:

  • Butorphanol
  • Pentazocine (Fortral)
  • Nalorphine
  • Diprenorphine (Revivon)
  • Buprenorphine (Temgesic)

The principal usage of opioids in medication is for analgesia. Analgesia is the loss of pain perception. Opioids effect both the physical and psychological perception of pain, physically blocking or raising the threshold of pain stimulation and removing the association of pain with fear. Associated with analgesia is sedation which is not considered hazardous, respiratory depression (which can also be associated with opioid analgesia) however can be a distressing side effect. A list of unwanted opioid effects includes:

  • Sedation
  • Excitement
  • Respiratory depression
  • Cough suppression
  • Nausea
  • Vomiting
  • Constipation

Opioid Selection

There are many opioids available for use, each with different properties, when selecting an opioid it is important to consider its potency, how quickly it acts (speed of onset) and how long it lasts (duration). The best analgesics are those which have a mild potency, rapid onset and a long duration of effect. When combining an opioid with a neuroleptic for neuroleptanalgesia, the desired properties of the opioid are slightly different; strong potency, rapid onset and brief period of duration.

As opioids can have an effect on the gastrointestinal system, (as opioid receptors are also found in the gastrointestinal tract) if they are to be given orally then they must have low lipid solubility.

Another point to consider is whether to use an agonist or an agonist-antagonist as both are able to produce analgesia. The main consideration is that pure agonists are more reliable and predictable than agonist-antagonists, but the agonist-antagonists produce fewer side effects such as vomiting, sedation and respiratory depression. Also as agonist-antagonists have antagonistic effects, any further use of analgesics may be compromised.

Below is a comparison of the potency of certain opioids relative to morphine, the most potent being Etorphine. Etorphine (or Immobilon) is extremely powerful and typically only used to immobilise large mammals (e.g. elephants). Due to its potency it can prove lethal to man.

Drug Relative Potency
Meperidine 0.1
Morphine 1
Butorphanol 1-2
Hydromorphone 10
Alfentanil 10-25
Fentanyl 75-125
Remifentanil 250
Sufentanil 500-1,000
Etorphine 1,000-3,000

Opioids are often used as part of a pre-medication routine i.e. before surgery as a pre-emptive form of analgesia. This is because once pain has been established (i.e. during surgery) pain relief drugs prove less effective. As a result larger doses would be needed to prevent the pain which increases the onset of associated side effects e.g. respiratory depression.

Examples of Opioids and their Properties:


Morphine (agonist) is considered the standard opioid with all other forms of analgesia being compared against it. It is the most potent natural analgesic, more potent derivatives have been artificial synthesised. Morphine produces a mixture of stimulant and depressant actions depending on the size of the dose as well as the species and absence or presence of pain.

Differences between the species can be observed e.g. in the dog, the cortex is depressed and little excitement is produced. In the cat, very small doses are able to induce excitement and in the horse morphine will not produce excitement if no pain is present (effect is less predictable in horses however). Despite this morphine is safe to use in all species as long as the correct dosage is used, the presence of excitement tends to increase with dose.

The duration of morphine is about 4 hours in all species, it is eventually metabolised by the liver. It is normally injected subcutaneously at a dose of around 0.1mg/Kg (in dogs and cats).

Use of morphine can either stimulate the medulla (which is followed by depression of the medulla) or directly depress the medulla.

Morphine has a number of effects on the gastrointestinal tract. Initially it may invoke vomiting and defaecation which is followed by constipation. Constipation is due to local effects on the small/large intestinal opioid receptors. Segmental tone of the intestines increases, along with sphincter tone but the action peristalsis decreases. This increases the time taken for intestinal contents to pass.

Other areas affected by morphine include:

  • The chemoreceptor trigger zone (CTZ) of the medulla is stimulated by morphine – this induced vomiting.
  • The occulomotor centre is stimulated which is responsible for producing miosis.
  • The cough centre is depressed – reducing coughing but making post-operation mucus accumulation a possible problem.
  • The vagal centre is stimulated, which increases gastrointestinal activity and is responsible for the initial defaecation. If a large dose is administered bradycardia may be induced (slowed heart rate <60bpm) by myocardium depression.
  • The respiratory centre is easily depressed by morphine, even with a low dose. This is due to a reduced response to elevated CO2 levels. The mechanisms involved in the regulation of respiratory rhythm are also affected – contributing to the overall depression of the respiratory centre.


Butorphanol (agonist-antagonist) is a widely used sedative and analgesic in dogs, cats and horses – combined with tranquillisers for sedation. It is around 1-2 times as potent as morphine, but it does have a slightly shorter duration of action at around 2-3 hours (Morphine – 4 hours). It has a much less profound effect on the respiratory system as the dose increases compared to morphine.

One major use of Butorphanol is for intravenous administration in the horse (0.1mg/Kg) to alleviate abdominal pain associated with torsion, impaction, intussusception (intestinal prolapse) and spasmodic colic.

Equine Colic

The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. Some examples are:

  • Spasmodic colic – Increased peristaltic contraction
  • Impactive colic – Caused by irritation to the lining of the bowel or ileum due to diet or ingestion of large amounts of sand/ dirt
  • Obstructive colic – Obstruction of the bowel by large food masses
  • Flatulent colic – Build of intestinal gases causing distension and pain
  • Parasitic colic – Intestinal pain from parasites such as roundworm or tapeworm
  • Idiopathic colic – From another cause which remains unknown

There are also many diagnostic tests for equine colic:

  • Increased heart rate with decreased circulating volume
  • Distinctive behavioural signs
  • Auscultation – Listening to internal body sounds
  • Abdominocentesis – The extraction of fluid from the peritoneum which can be useful in assessing the state of the intestines
  • Nasogastric Intubation – Insertion of a tube from the nose to the stomach which can be used to drain excess liquid from the stomach – for therapeutic reasons and for diagnosis
  • Rectal/Faecal examination

There are also many drugs/treatments available to treat the symptoms:

  • Analgesics
  • Spasmolytics
  • Lubricants/laxatives
  • Antizymotics – Used against disease producing organisms e.g. bacteria
  • Anthelmintics – Used against parasites
  • Fluid therapy

The major analgesics used against colic are α2-agonists (xylazine, romifidine and detomidine), opioids (butorphanol) and NSAIDs (flunixin).

Butorphanol is usually used alongside small doses of xylazine, romifidine and detomidine. This is because it has minimal effects on the cardiovascular system (which is not true for xylazine, romifidine and detomidine). Both butorphanol and the α2-agonists have a duration of around 2-3 hours and they both reduce intestinal motility/activity.

Non-Steroidal Anti-inflammatory Drugs (NSAIDs)


NSAIDs are non-narcotic analgesics (An analgesic reduces or removes the sensation of pain), they are also anti-pyretic (fever) and anti-inflammatory. These effects are produced by the inhibition of the fatty acid cyclooxygenase (COX) which inhibits prostaglandin synthesis.

Because NSAIDs are non-narcotic they do not cause any largely noticeable effects on the CNS (central nervous system) function. This makes them ineffective against normal nociceptive tests, these are test designed to test pain responses in living organisms and they are specifically used in the testing of new analgesic drugs. Methods usually involve the applying of pressure to a specific point of the organism. NSAIDs only raise the pain threshold when pressure is applied to a swollen and inflamed joint (this is known as analgesia via peripheral mechanisms). Therefore NSAIDs are considered anti-inflammatory agents with a mild central analgesic effect (associated with anti-pyretic action). NSAIDs are therefore primarily used in the treatment of acute or chronic conditions producing mild-moderate pain, especially involving the musculo-skeletal system. Principal utilisation occurs in the horse and dog.

Another benefit of NSAIDs non-narcotic function is that, by having an unnoticeable effect on the CNS they can therefore be used as a pre-med drug, before general anaesthetic, without fear of overloading the CNS. The use of an analgesic before the introduction of pain means that a lower dose will be required when pain is inflicted – thus reducing the chances of side effects associated with high doses. However, the type of NSAID must be selected for carefully as there may be a possibility of renal damage/toxicity. There are only a couple of NSAIDs which are believed to be renal safe.

Prostaglandins and Inflammation

NSAIDs work by inhibiting prostaglandin synthesis by targeting the COX enzyme. Prostaglandins activate the inflammatory response giving the production of pain and fever, they are produced when leukocytes reach a site of damaged tissue in an attempt to minimise tissue destruction.

Prostaglandins are involved in several other organs such as the gastrointestinal tract (inhibit acid synthesis and increase secretion of protective mucus), increase blood flow in kidneys, and leukotrienes which promote constriction of bronchi associated with asthma.


What is/are the precursor(s) of prostaglandins?

The answer may be found in the ‘Mechanisms of NSAID Action’ section.

Cyclooxygenase Enzymes (COX)

COX forms two isoforms, COX-1 and COX-2:

  • COX-1 is often thought of as being the ‘good’ COX; this is due to its involvement in tissue homeostasis. It is required to keep the body ‘normal’ – primarily the synthesis of prostaglandins responsible for protection of the stomach lining. (Constitutive physiological).
  • COX-2 therefore is thought of as the ‘bad’ COX; this is because it is produced during inflammation, by the inflammatory cells which have been activated by cytokines. (Inducible physiological).
  • There is some evidence for a COX-3, a possible variation of COX-1 which is also associated with the inflammatory response. It has been found in the CNS and is affected by paracetamol.

Mechanism of NSAID Action

The primary action of NSAIDs is the inhibition of the COX enzyme, by inhibiting this enzyme the production of prostaglandins are also inhibited. The COX enzyme synthesised prostaglandins from fatty acids such as arachidonic acid.

Most NSAIDs inhibit both major forms of the COX enzyme, however all are still considered toxic. Newer drugs which are believed to be COX-2 specific (thereby not affecting the COX-1 enzyme and allowing prostaglandins associated with normal function to continue normal operation) are relatively safer in chronic use. There are fewer side effects which is what makes them be suited to prolonged periods of use. Examples: Merck’s rofecoxib and etoricoxib, Pfizer’s celecoxib and valdecoxib.

The NSAIDs selective for COX-2 are now however under scrutiny, due to reports of cardiovascular toxicity. These include strokes and myocardial infarctions. This has resulted in the withdrawal of certain COX-2 selective drugs such as rofecoxib. Further studies are suggesting that the cardiovascular toxicity of these COX-2 selective NSAIDs may actually not be much greater than ‘trusted’ NSAIDs such as ibuprofen. However, their toxicity still remains a point of research and discussion.

Other Actions of NSAIDs

Besides from the inhibition of the COX enzyme, other actions include:

  • Inhibit superoxides (toxic) and free radicals
  • Inhibit Bradykinin production (A Peptide which dilates blood vessels, lowering blood pressure)
  • Stabilises lysosomes
  • Inhibits metalloproteinases (Proteolytic enzymes whose catalytic mechanism involves a metal)
  • Antagonises interleukin-1 (fever inducer and controlling factor of lymphocytes) and tumour necrosis factor (TNF – cytokine involved in the induction of inflammation and apoptosis, dysfunction of this factor is believed to be involved with the production of cancers.)


This is the half maximal inhibitory concentration (IC50). It is a measure of the effectiveness of a compound (typically a drug candidate) in inhibiting biological or biochemical function. This quantitative measure indicates how much of a particular drug is needed to inhibit a given biological process (or component of a process, i.e. an enzyme, cell, cell receptor or microorganism) by half. It is commonly used as a measure of antagonist drug potency in pharmacological research.

This can be related to inhibition of COX enzymes, we can use it to find out how many times a dose required to inhibit COX-1 we need to administer in order to inhibit COX-2, i.e. COX-2/COX-1,  a lower ratio is better as it shows that the drug has a higher selectivity for COX-2 the ‘bad’ COX associated with inflammatory responses.

COX-1 Selective               Drugs                       Amount times dose required to inhibit COX-2

  • Aspirin                                                                  170
  • Piroxicam                                                             250

Less Selective COX-1 Drugs

  • Paracetamol                                                            7
  • Ibuprofen                                                                15


  • Naproxen                                                                  1

COX-2 Selective Drugs

  • Meloxicam                                                               <1

The Anti-Pyretic Effect of NSAIDs

NSAIDs do not affect normal body temperature only when pyrexia (fever) is present, do they alter temperature. Bacterial or endogenous (substances from within the body) pyrogens can act directly on the hypothalamus. Heat temperature is regulated in the hypothalamus by controlling temperature regulating peripheral mechanisms such as vasoconstriction/dilation, sweating, shivering and metabolic activity.

Pyrogens activate hypothalamic COX, increasing prostaglandin concentration. The effect of this is that the set body temperature (Average 37oC) is increased (Pyrexia is considered >38oC). NSAIDs counter this by inhibiting prostaglandin synthesis by the inhibition of COX enzymes. By doing this they have effectively blocked the action of the pyrogens on the CNS and return the raised set body temperature back down to normal levels (37oC).

Commonly Used NSAIDs

Some of the most commonly found NSAIDs and their properties:

Aspirin (Acetylsalicylic acid)

Aspirin is a potent anti-inflammatory drug with mild central analgesic and antipyretic actions. It is administered orally and readily absorbed from the stomach and small intestine, an acid drug is well absorbed in an acidic environment. It is metabolised by tissue /plasma esterases. Aspirin may also be used in low doses, daily to prevent platelet aggregation.

In a healthy body, thromboxane and prostacyclin (eicosanoids – fatty acid signalling molecules) are balanced. Aspirin however disrupts this balance in the favour of prostacyclin, inhibiting aggregation.

Aspirin irreversibly binds to platelet COX, however as platelets are produced every few days, the condition is not permanent, and this is why chronic dosing may be necessary.

Aspirin is toxic in cats due to their lack of the enzyme UDP-glucuronyl transferase, therefore when giving aspirin to cats the maximum stated dose is 25mg/kg daily (Compared to 25mg/kg 3-4 times a day in dogs) Toxicity effects may still appear even at lower doses e.g. vomiting, abdominal pain, anorexia and gastric ulceration.


Paracetamol is a weak anti-inflammatory drug; however it does have a more potent central analgesia and anti-pyretic effects than aspirin (See IC50 table to see a smaller dose is required of paracetamol to inhibit COX-2, the COX enzyme responsible for inflammatory responses).

Due to paracetamol being well tolerated, producing less gastric irritation than aspirin and having much fewer side effects than aspirin, it has become a predominant household analgesic. Acute overdoses can cause fatal hepatic damage; early symptoms include anorexia, vomiting, diarrhoea and abdominal pain. It is the reactive metabolites of paracetamol latching onto -SH groups that cause hepatic toxicity.

The dog is more resistant to paracetamol toxicity than cats, an oral dose is recommended every 6 hours of 25-30mg/kg.


Phenylbutazone is the most widely used NSAID in equine medicine; however it is extremely toxic in humans. It has a long  t1/2 (half-life) of 70 hours and produces severe gastric ulceration and agranulocytosis. It can be administered orally and by intravenous injection. Due to the acidity of the drug, it is readily absorbed from the stomach/duodenum. Phenylbutazone metabolites are weak acids and therefore preferably excreted in alkaline urine. Training horses may have acidic urine, and so it is recommended not to take Phenylbutazone within 8 days of competition.

Half-life in dogs of Phenylbutazone has been recorded at 3-8 hours (however this may vary dependent on the dose). Phenylbutazone can inhibit the synthesis of prostaglandin in inflammatory exudates for 12-24 hours, with the response lasting for up to three days after the final dose in the course.

Signs of Phenylbutazone toxicity include inappetance and depression with weight loss and oedema. The oedema (fluid retention) is due to the decreased NaCl excretion.

Dosing of Phenylbutazone should not exceed:

  • Day 1 – 4.4 mg/kg twice a day
  • Day 2-4 – 2.2 mg/kg twice a day
  • Day >5 – 2.2 mg/kg daily

Phenylbutazone is also administered in combination with other drugs for the treatment of musculoskeletal disorders e.g. Tomanol – Phenylbutazone and Isopyrin

Meclofenamic Acid (Arquel)

Meclofenamic acid is a potent anti-inflammatory, anti-pyretic analgesic. It is more potent than aspirin but similar in effect. As well as inhibiting COX enzymes it has found to be a prostaglandin antagonist, interacting with prostaglandin receptors. It therefore prevents the action of prostaglandin already present possibly exerting a more rapid reduction of inflammation.

Half-life is 6-8 hours; therapeutic levels are maintained with daily doses.

Unlike other NSAIDs onset of Meclofenamic acid is relatively slow, taking 36-96 hours for effects to begin

Prolonged administration in the horse may lead to anorexia, depression, ulceration of buccal mucosa, gastric haemorrhage or diarrhoea


Naproxen is a propionic acid derivative (like ibuprofen or Ketoprofen) with a tendency for reduced frequency of serious side effects at therapeutic doses. In the horse, half-life of the drug is 5 hours; double daily doses have been proven effective in soft-tissue inflammatory conditions such as myositis. In the dog however, half-life is much longer at around 70 hours, it is therefore recommended to avoid this NSAIDs due to excessive toxicity


Ketoprofen is a potent COX inhibitor which is also able to stabilise lysosomal membranes and is a Bradykinin antagonist. It is also reported to be an inhibitor of the lipoxygenase enzyme system (iron-containing enzymes which catalyse the dioxygenation [incorporation of two oxygen atoms] of some polyunsaturated fatty acids). It is around 50x more potent than Phenylbutazone, however this is not accompanied by an equivalent increase in toxicity. It is also suggested that it may be cartilage sparing, neither accelerating chondrocyte damage nor reducing proteoglycan production.


Carprofen is a potent anti-inflammatory drug, but is a weak inhibitor of COX. Its mode of action is not yet known but it significantly inhibits neutrophil migration. Due to weak inhibition of COX, toxicity of Carprofen its toxicity tends to be low.


Piroxicam is a potent and long lasting anti-inflammatory drug. Its half-life of around 60 hours enables lower dosing (alternate days). A higher frequency of dosing will produce standard NSAID toxic effects (see below, section ‘Side Effects of NSAIDs’)


Meloxicam is a similar drug, but with a shorter half-life (30-40 hours). It is thought to have greater potency for COX-2 than COX-1 therefore side-effects may be less. It is also thought to be chondroprotective (The slowing of degradation of articular cartilage)


Flunixin is a potent versatile anti-inflammatory drug with a short half-life in all species (2-8 hours). However its duration of action is relatively long (24-36 hours)

When to Use NSAIDs

  • Mild to moderate inflammatory lesions and associated pain
  • Acute inflammation and pain
  • Joint inflammation and pain
  • Suppression of pulmonary oedema
  • Endotoxaemia
  • Anti-thrombic

Side Effects of NSAIDs

  • Gastric irritation and ulceration – This is a main side effect of chronic NSAID use, it occurs because when NSAIDs inhibit the COX-1 enzyme, COX-1 synthesises prostaglandins associated with inhibiting acid synthesis and increasing secretion of protective mucus. SO by inhibiting COX-1, the stomach becomes unprotected from the gastric acid causing irritation and possibly ulceration in chronic use.
  • You can protect the gut by administering proton pump inhibitors (Losec), prostaglandin analogues (Misoprostol) or H2 receptor antagonists (Zantac)
  • Vomiting and diarrhoea
  • Hepatotoxicity
  • Renal papillary necrosis, chronic nephritis
  • Bone marrow disturbance
  • Skin rashes
  • Respiratory distress