Hard to say, and that may be the biggest trouble with tribulus terrestris. The United States Food and Drug Administration does not regulate supplements, like tribulus terrestris, so it has not been examined thoroughly as drug treatments and medicines have. Because of this, the full range of side effects from tribulus terrestris remains a mystery right now, even though there are some sites claiming there are no known side effects.
One possible adverse side effect may be an increase in testosterone levels. While good for some purposes, boosted testosterone levels can cause rage, affect prostate size and hair growth. If you do decide to take tribulus terrestris, please take a supplement and avoid ingesting any part of this prickly plant. One man tried to do so and wound up puncturing a lung.
And, as with all supplements, not all are created equally. One proponent suggests insisting on a tribulus terrestris supplement with at least 45 percent protosioscin, the active ingredient. And remember, consider whether this might react with any other medication you might be taking. And, as always, consult your medical professional first.
source: http://www.sharecare.com/question/tribulus-terrestris-side-effects
