Tribulus terrestris: Ayurvedic tonic and aphrodisiac; used in European folk medicine to increase sexual potency. Chief constituent: protodioscin, a steroidal saponin. In animal studies, Tribulus increased sex hormones (e.g., testosterone) and improved nitric oxide synthesis. Explanation: differences in extract and plant parts and the fact that studies included healthy males with normal testosterone. Tribulus enhances male fertility by increasing sperm count, viability, and libido