verb

administer drugs to (a racehorse, greyhound, or athlete) in order to inhibit or enhance sporting performance.

the horse was doped before the race

drug,tamper with,interfere with,disable,stupefy,sedate,befuddle,inebriate,intoxicate,narcotize,incapacitate,weaken,knock out,anaesthetize,give an anaesthetic to,make/render unconscious,nobble