The development of breast tissue in young men (gynecomastia) goes by a lot of unflattering names such as man boobs and bitch tits. For the overwhelming majority of men affected, there is no identifiable cause. Clearly there is some hormonal influence but the exact trigger is not clear for most patients. Some of the young men that I treat for gynecomastia often sheepily say that they smoke marijuana and that is why they probably developed it. That belief is well known and frequently stated as fact but is it really true? The unequivocal answer is…maybe. The main active ingredient in marijuana is THC which has been shown to affect testosterone production. Presumably this could cause a man’s estrogen levels to increase causing a stimulatory effect on breast tissue. But what levels of THC are needed and how long does it take to cause this adverse effect? No one knows for sure and how a man may be affected is undoubtably different. There also other compounding factors such as being overweight, taking steroid or muscle building supplements and using Propecia for hair loss, all of which can also decrease testosterone levels. Gynecomastia is a multifactoral problem as many men smoke marijuana and never develop gynecomastia while a few do. It is a risk factor but its inhalation does not assure that gynecomastia will develop.