Taking a lot more testosterone than your body normally makes—without working out—can make your muscles bigger and add about 7 pounds of muscle mass, showing that testosterone alone can build muscle.

If a guy who already lifts weights takes a high dose of testosterone and keeps training hard for 10 weeks, he’ll gain a lot more muscle and strength than if he just took the hormone or just trained alone.

If a guy who already lifts weights takes a high dose of testosterone for 10 weeks without working out, he’ll get stronger on the bench press and squat—meaning testosterone alone can make muscles stronger, even without training.

If young men eat much less and exercise a lot for four weeks while taking a high dose of testosterone, they gain a few extra kilograms of muscle compared to those taking a fake pill—even though their strength and leg function don’t get better.

Taking a high dose of testosterone for 10 weeks doesn’t make healthy male weightlifters feel more angry, moody, or act differently, according to standard mood tests and observations by others.

Trans guys who aren't athletes gain noticeable muscle and strength in their first year on testosterone, and after about 3 years, their push-ups, sit-ups, and running get close to or match the average fitness levels of cis men.