Gynecomastia is caused by a disruption in hormone balance. In most cases, gynecomastia is linked to the use of anabolic steroids or androgens. It can also be caused by using illegal drugs, like heroin and marijuana. Gynecomastia may also be caused by certain health conditions (hypogonadism, hyperthyroidism, and tumors), plus some medications (highly active antiretroviral therapy for AIDS, anti-anxiety medications, and chemotherapy).