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.

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 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.

When healthy men take a high dose of testosterone for 10 weeks, their body shuts down some of its own hormone production—like turning off a faucet—because it thinks it already has enough.

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 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.