Another cause of the problem can be prescription or over-the-counter medications. In fact, over 200 FDA-approved drugs list ED as a side effect. Drugs that may cause problems in the bedroom include some antidepressants, like Prozac and Zoloft, diuretics like Lasix, antihistamines such as Benadryl and muscle relaxants like Norflex. Medications that men take for chemotherapy, prostate cancer and Parkinson's disease can also result in ED.