Assessing your horse’s response to the use of pain-relieving medication (e.g. “bute” trial)

“It is important to be aware that improvement in performance and behaviour during medication and/or deterioration in performance after stopping medication are positive indicators of the presence of pain. However, a negative response to phenylbutazone does not preclude the presence of pain, because phenylbutazone is not effective in relieving all sources of pain.” - Dr. Sue Dyson

There are many different types of non-steroidal anti-inflammatory analgesic drugs (NSAIDs) available for use in horses. In my experience phenylbutazone (‘bute’) is the most effective drug for the relief of musculoskeletal pain and is the best drug for a ‘pain test’ provided that it is administered for long enough at a suitable dose rate, which is higher than the dose rate typically recommended for treatment. Although phenylbutazone is highly toxic to people, horses generally tolerate the drug well, with no side effects.  In the rare event that a horse develops diarrhoea after the initiation of the trial, no further phenylbutazone should be given. A ‘bute trial’ is best done in collaboration with your veterinarian.

Phenylbutazone is not particularly palatable and is best given mixed in a small dry feed. Alternatively, it is available in paste form, or the powdered format can be administered as a suspension by syringe.

I recommend keeping a diary of a horse’s daily ridden performance for 7 days prior to initiation of the trial. This could include application of the Ridden Horse Pain Ethogram (RHpE, otherwise known as the Ridden Horse Performance Checklist). Continue to keep a daily diary during the trial period (7-10 days), while the horse is in normal work, and for 7 days after the trial. Deterioration in performance after the trial period is an indication that phenylbutazone did influence the horse’s performance even if improvement in performance during the trial period had not been recognised.

Phenylbutazone, like most drugs given to horses, should be administered on a bodyweight basis (4.4.mg/kg twice daily), equivalent to approximately 2g (two sachets) twice daily for a 500kg (1,000lb) horse. 

It is important to be aware that improvement in performance and behaviour during medication and/or deterioration in performance after stopping medication are positive indicators of the presence of pain. However, a negative response to phenylbutazone does not preclude the presence of pain, because phenylbutazone is not effective in relieving all sources of pain.

Copyright Sue Dyson January 2024