The National Collegiate Athletic Association (NCAA) lists synephrine (bitter orange) as a banned stimulant. One small study found that taking 100 mg of synephrine slightly enhanced muscle endurance during resistance exercise compared to control.[1]
A longer-duration study found that 4 weeks of daily administration of a supplement containing 20 mg synephrine improved some areas of exercise performance, but after 8 weeks no clear differences compared to the supplement without synephrine were noted.[2]