Although protein powders rarely contain substances banned in sports, it's vital for athletes to be cautious about which protein powder they choose. Some pre-workout supplements, for example, may contain stimulants prohibited in athletic competitions, particularly if they have a "proprietary blend" of sorts. To avoid any risks of accidentally consuming banned substances, we'd recommend athletes go for protein powders that are certified by Informed Sport, Informed Choice, or NSF Certified Sport. These certifications ensure the products have been tested and are free of banned substances.