Incubation of sperm with curcumin caused a concentration-dependent decrease in sperm forward motility, capacitation/acrosome reaction, and murine fertilization in vitro. At higher concentrations, there was a complete block of sperm motility and function within 5-15 min. Administration of curcumin, especially intravaginally, caused a significant (P < 0.001) reduction in fertility. The antifertility effect of curcumin was reversible. This is the first study to report the inhibitory effect of curcumin on sperm function, fertilization, and fertility. The findings suggest that curcumin may constitute a double-edged sword to block conception, infection, and cancer, thus providing an ideal contraceptive.