There is currently not enough research to say whether Coleus forskohlii affects testosterone.
Specifically, only one trial has looked at the effects of supplementation with Coleus forskohlii on testosterone levels.[1] In this trial, 30 men with overweight/obesity and normal testosterone levels took 500 mg of Coleus forskohlii extract (containing 10% forskolin) daily for a period of 12 weeks. At the end of the trial, the levels of free testosterone increased in the Coleus forskohlii group by 3.5% and decreased in the placebo group by 4.1%, with the difference between groups reaching statistical significance. No effect of supplementation with Coleus forskohlii extract was observed on the levels of total testosterone.