If you use a ****load of it for a long time consistently it can lower your testosterone. This based on animal studies I remember reading when I was an activist on the issue. However, even in the megadosed studies testosterone only went down slightly and was still well within the normal range.
In other words, if you have a rat don't feed him x times the human psychoactive dose for his entire life, or he may feel *****y towards the end.
NEW STUDY FINDS POT DOESN'T LOWER TESTOSTERONE
A new study by Dr. Robert Block at the University of
Iowa disputes the commonly held notion that marijuana alters
the level of testosterone and other sex hormones.
The study contradicted a widely publicized 1974 study
by Dr. R.C. Kolodony, which reported decreased testosterone
levels in men who smoked marijuana chronically.
The U. of Iowa study found that chronic marijuana use
had no effect on testosterone, luteinizing hormone, follicle
stimulating hormone, prolactin and cortisol in men or women.
Noting that six other studies had failed to show
lowered testosterone levels in men, Dr. Block concluded: "It
appears that marijuana, even heavy use of the kind that's
typical in the United States, doesn't alter testosterone levels."
However, he cautioned that heavy use might have other
adverse effects, including "possible effects on reproductive
function and mild, selective cognitive impairments associated
with heavy, chronic use."
Block's study is published in Drug and Alcohol
Dependence, Vol. 28: 121-8 (1991).