Hi Lee,
The NIH Center for Complementary and Alternative Medicine do list some cautions with Yohimbe. These include headaches, anxiety, sleepiness, dizziness, and a general caution about over use if taken in large doses. It shouldn't be taken if you are on antidepressant medication or the phenothiazine group of medicines as used for certain mental health treatments. Anyone with kidney problems is also advised to steer clear.
From what I can see, the effectiveness of Yohimbe has been met with some mixed reviews. Here's a link to MedlinePlus on the topic.
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