Tongkat Ali (Eurycoma longifolia) is among the better-studied natural testosterone-support herbs, with human trials showing increases in free testosterone and reductions in cortisol, particularly in men who start with below-average levels — the effect is far less pronounced in men with already-healthy testosterone. Look for extracts standardized to eurycomanone content (the most-researched active marker) and dosed from root rather than leaf extract, since potency and quality vary widely across the supplement market. It is commonly stacked with other test-support botanicals, though stacking multiple hormonal herbs compounds the uncertainty around any single ingredient's contribution.