Propantheline bromide, or "Blue Magic," increases blood flow to muscles by acting as a relaxer. Charges have been brought in several cases for its use in harness racing.
Narcotic analgesics are intended for pain relief but they also act as a stimulant in horses.
Etorphine, or "Elephant Juice," is a large animal tranquilizer. It acts as a stimulant rather than as a tranquilizer when used in horses in the right doses.
Butazolidin is anti-inflammatory and pain-killing so it would be used to improve the performance of a horse with pain due to an injury.
Bicarb is common baking soda. It absorbs lactic acid, which is a waste product of muscle exercise, allowing the horse to sustain the same level of exertion for a longer period of time. Bicarb is allowed up to a threshold level.
Caffeine used to be a popular stimulant but it is easily detected by modern tests so it has given way to newer drugs.
EPO (erythropoeitin) hasn't emerged onto the harness racing scene as it has in the human scene but it is prohibited.
Anabolic steroids became very popular in the late 1980s but have mostly become a thing of the past because they are so easily detected with current testing methods.
Two New Zealand harness trainers were temporarily disqualified in September 2009 for presenting horses dosed with aminocaproic acid, a medication used to control bleeding in human hemophiliacs.