Trainers will dope their horses to mask a physical defect or to increase speed and performance of the animal (Sanford 1974).

Stimulants

• Caffeine
•  Amphetamine
• Cocaine
• etc..

Stimulants are the most common drugs given to horses competing in all disciplines.  A stimulant is administered to increase the horses performance and speed by increasing alertness and removing muscle fatigue, although it has not been proven that this will occur in a fit horse (Winter 1951).

Diuretics 

• Athiazide
• etc…

Diuretics are given to reduce the blood pressure of the horse. The blood vessels in the lungs are very thin, as the horse exercises and blood pressure increases these vessels are at risk of bursting. If they burst blood will pool in the lungs making it hard for the horses to breathe.

Lasix-4 is a diuretic that is not banned in the United States or Canada and is commonly used. Here is a link to a short video explaining the controversy surrounding this medication. People think that the use of this drug has moved past improving welfare into performance-enhancing.

Lasix-4 Debate

Beta-Blockers

• Timolol
• Sotalol
• Butofilolol
• etc…

Beta-blockers block the stress response in animals, particularly the fight or flight response.  This allows them to stay calm and focused on the task at hand.

Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)

• Pyroxicam
• Rofecoxib
• etc…

NSAIDs block the immune response and allowing the horses body to function past its limits. This is especially important in areas such as the lungs, decreasing bronchiole inflammation allows for increased oxygen delivery. In general, reducing inflammation decreases the pain response so the horse can perform past its limits.

Analgesics

Analgesics reduce the pain response.

Vasodilators

Vasodilators dilate blood vessels allowing for increased blood flow to working muscles.

Anticoagulants

Anticoagulants thin the blood, reducing blood pressure and allowing for easier flow to the muscles.

Anabolics

Anabolics repair damaged tissue.

Sedatives

Sedatives calm the horse and can be used for nervous horses.

Muscle Relaxants

Muscle relaxants calm the horse and decrease tenseness.

Antidepressants

Antidepressants decrease the anxiousness of the horse.

Antihistamines 

Antihistamines reduce swelling and the pain response.

etc…

There is a huge variation in the drug types given to horses. There are many more categories of prohibited substances that all work to reduce pain, swelling, nervousness, fatigue, etc.