The truth? While research is limited, there’s nothing to suggest that deer antler velvet (or deer antler spray in the supplement form) actually does what it claims. In fact, there are two published studies (in real scientific journals, you can see them here and here) that suggest deer antler velvet does not (I repeat, does not) even elicit a hormonal response. What’s more, it also did not increase muscular strength or aerobic power.
Generally researchers agree that deer antler velvet protects, strengthens and restores the body's functions that are out of balance. In other words, it can act as an adaptogen on the human body, helping where it is needed. Studies suggest that deer antler velvet may have beneficial effects related to: increases in muscular development, strength and endurance; improved recovery; prevention and reduction of inflammation; improvement of sexual health; reduction in blood pressure; improvement in bone and joint health; stimulation of the immune system; and more.
A systemic review on human interventions makes note of a study conducted on patients of osteoarthritis (Edelman et al. 2000; cannot be located online) which found improvements in joint pain symptoms relative to baseline in the Velvet Antler group and not placebo, although a lack of information on blinding and randomization precludes results that can be drawn from this study.