According to WebMD, the effects of a Botox injection last four to six months. A shot of Botox blocks signals that transmit from the nerves to the facial muscle into which it was injected, which means the muscle cannot contract. Botox forces the muscle to relax, making facial wrinkles soften.
As the effects of the Botox injection fade, the ability of the facial muscle to contract gradually returns, which results in the reappearance of wrinkles. Since the facial muscles are being taught to relax, lines and wrinkles often look less pronounced over the course of a series of repeated injections, according to WebMD.