For men over 45 taking testosterone therapy, their blood often gets thicker with more red cells, making it almost four times more likely to reach unsafe levels—so doctors need to check their blood regularly.

Men over 45 who take testosterone to boost low levels may be 78% more likely to have a prostate problem detected than those who don’t, so doctors should keep a close eye on them during treatment.

For men with low testosterone, taking testosterone therapy might slightly raise the number of red blood cells, but it doesn’t seem to make blood clots or lung blockages any more likely over almost two years of use.

For men over 45, taking testosterone supplements doesn't seem to make heart problems, sleep apnea, or dying any more likely than not taking them.

Studies with lots of people and strict methods show that giving men testosterone at normal body levels doesn’t make them more likely to die or have serious heart problems.

For men between 55 and 67, taking testosterone therapy doesn’t seem to make them more likely to die from any cause over 18 years — their risk is about the same as men who don’t take it.