ATCvet code: QN02AF01

Butorphanol is a centrally acting opioid analgesic with agonist-antagonist activity at opiate receptors in the central nervous system. Activation of opioid receptors is coupled to changes in ion conductance and G protein interactions which lead to inhibition of pain transmission. Butorphanol has agonist activity at the kappa (κ) opioid receptor subtype and antagonist activity at the mu (µ) opioid receptor subtype. The agonist component of butorphanol activity is ten times more potent than the antagonist component.

Butorphanol as a sole agent provides dose-dependent analgesia and can also cause sedation (horses and dogs). Butorphanol in combination with certain alpha-2 adrenoceptor agonists results in profound sedation and in combination with certain alpha-2 adrenoceptor agonists and ketamine results in anaesthesia.

Onset and duration of analgesia:

Analgesia generally occurs within 15 minutes following intravenous administration. After a single intravenous dose in the horse, analgesia usually lasts for 15-60 minutes.

The volume of distribution after intravenous injection is large suggesting wide distribution into tissues. The volume of distribution is 7.4 L/kg in cats and 4.4 L/kg in dogs. Butorphanol is metabolised extensively in the liver and mainly excreted in urine.

In the dog, after intramuscular administration butorphanol has a high clearance (around 3.5 L/kg/hour) and a short terminal half-life (mean < 2 hours). This indicates that, on average, 97% of an intramuscular dose is eliminated in less than 10 hours.

In the cat, after subcutaneous administration, butorphanol has a relatively long terminal half-life (around 6 hours). This indicates that, on average, 97% of a subcutaneous dose is eliminated in approximately 30 hours.

In the horse, after intravenous administration, butorphanol has a high clearance (on average 1.3 L/kg/hour) and a short terminal half-life (mean < 1 hour).This indicates that, on average, 97% of an intravenous dose is eliminated in less than 5 hours.