Testosterone can push your red blood cell count too high
Across four placebo-controlled trials in 1779 men, an overproduction of red cells that thickens the blood was 8.14 times more likely on testosterone than on placebo.
What the study found
Testosterone made the blood eight times more likely to thicken
Across four placebo-controlled trials in 1779 men, erythrocytosis, an overproduction of red blood cells that thickens the blood, was 8.14 times more likely on testosterone than on placebo. Guidelines list an already-raised red-cell count among the reasons not to start at all.
The caveat · Too few cases to pin the size down
The relative risk of 8.14 runs across an interval from 1.87 to 35.40, which reflects how few men had the event rather than a precise estimate. It rests on four trials, and those trials needed to run only twelve weeks, short against how long testosterone is actually taken.