Pharmacokinetics of intramuscularly administered morphine in horses.

The research focused on understanding how morphine, when given intramuscularly, behaves in a horse’s body. It confirmed that the morphine’s half-life is short, around 1.5 hours, hinting that repeated doses might be necessary to maintain effective concentration levels. However, the optimal dosage might be different for each horse.

Research Methodology

• The study was initially carried out prospectively with 77 horses taking part.
• Morphine sulfate was given to the horses at a dose of 0.1mg/kg intramuscularly.
• After the morphine injection, blood samples were collected at predetermined time intervals to measure the concentration of morphine in the horse’s blood.
• In a preliminary study, two healthy horses were observed for up to 12 hours after receiving the morphine dose.
• In the main clinical phase, blood samples were obtained from 75 hospitalized horses at various times up to 9 hours after drug administration.
• Plasma morphine concentrations were measured via liquid chromatography and mass spectrometry.

Findings

• The research found that the terminal half-life (t(½)) of morphine was approximately 1.5 hours.
• The volume of distribution per bioavailability was found to be around 4.5 L/kg.
• The clearance per bioavailability was approximately 35 mL/kg/min.
• The highest plasma concentration was 21.6 ng/mL, which occurred approximately 4 minutes after administration.
• Morphine concentrations dropped below the limit of quantification in 74 horses within 7 hours post-administration.
• Adverse effects were believed to be caused by morphine administration, yet these were rare and mild in nature.

Conclusions

• The short half-life of morphine suggests that frequent administration would be necessary if a consistent effective plasma concentration is to be maintained in horses.
• Additionally, the research noted fluctuations in plasma concentrations hinting at possible variations in the ideal dosage for different horses.
• Despite the generally good tolerance at the tested dosage, careful monitoring is still recommended when horses are given morphine.