Does propecia cause gynecomastia often, or is it very rare?

According to official statistics, 36% of adult young men and 57% of older #adult men in the United States have this medical condition in one of the four grades. About 200,000 to 3 million cases of #gynecomastia are being reported every single year. So if you have #breast #enlargement and you feel you're an exception, please understand that 1 in 3 young adult #men and 1 in 2 older adult men are just like you and have breast enlargement. Several common factors can cause gynecomastia including excessive levels of #estrogen, natural #hormone changes, use of recreational drugs or alcohol, medications and their associated side effects, and, various health conditions. Also, there are different types of #gynecomastia.There are three types of surgical #treatment #options available that treat gynecomastia. A board-certified plastic surgeon will recommend the type best suited based on the individual. The most important decision to be made before performing any surgical procedure is determining whether who is an ideal #candidate.

Gynecomastia is a known rare side effect of finasteride 1mg (Propecia)  There are statistics of this in the drug insert and papers available on the Internet. 

Gynecomastia or enlargement of breast tissue seems more common at the 5 mg dose than the 1 mg dose. The enlargement may be associated with pain or tenderness. Overall, estimates suggest that out of every 1000 users of this medications, about 4-10 will develop gynecomastia. Breast tissue enlargement occurs rapidly or in a delayed manner - ranging from 14 days to 2.5 years in some studies. One side is involved in 50 % of patients and both sides are involved in the other 50 %. If you are concerned about gynecomastia from use of this medication, be sure to speak to your physician about risks and benefits and how to monitor. REFERENCEGreen et al. Gynecomastia and Breast Cancer during Finasteride Therapy. N ENGL J MED 1996; 335; 823

The relationship between gynecomastia and propecia is known, but fortunately rare. I would guess its appearance is about 1:1000 patients taking propecia. When it happens, it can be a problem as stopping the drug often does not reverse the condition if breast nodules appear. If you have such a problem, go see your doctor