Mana is continuously produced by the body in order to keep a person alive and functional. It is also kinked to magical potential. Testosterone is an important signalling hormone in a mana production pathway. Physically large and athletic young men produce the most Testosterone. Mana builds up over time and can be boosted before harvesting after a testosterone spike (battle, ritual dancing, sex, etc).This would naturally make fit young males the most powerful warlocks in the setting.
Steroids are synthetic variations of the male hormone testosterone, and the human body can't tell the difference. Athletes and bodybuilders abuse these drugs to boost performance or improve their physical appearance. Since testosterone is being increased, it stands to reason that those who use it would increase their magical potential. However, it has no effect on ability and in some cases even decreases it. Why would this be the case?