A common cause of large breasts (gynecomastia) in men is an increased sensitivity of the breast tissue to drugs. There are many drugs which are known to cause gynecomastia. These include:
marijuana
valium(diazepam)
amphetamine,
aldomet
cimetidine,
digitalis
spironolactone
androgens,
estrogens,
isoniazid
phenothiazines
tricyclic antidepressants
cytotoxic drugs
If you are an adult man and have developed larger breasts, a first step may be to discontinue the drug. Any change in your medications should only be made after talking with your doctor. There are many other causes of gynecomastia and your doctor may suggest further tests.