Yohimbine

Yohimbine: This herbal product has been available for many years. It comes from the bark of a West African tree. Its use has been questioned because, even in good, well-controlled studies, yohimbine is only slightly better than placebo (no drug at all). A renewed interest in this agent has occurred, particularly when combined with sildenafil or some of the other oral drugs. Yohimbine is safe with few known adverse effects. The customary daily dose is one tablet of 5.4 mg taken 3 times a day. SOURCE: webmd.com

While this article notes that Yohimbine is "safe" I have read many articles that suggest at least some risk... always consult a physician.