Side effects with Deca-Durabolin are relatively low with dosages of 400 mg/week, androgenic caused side effects can occur. Most problems manifest themselves in high blood pressure and a prolonged time for blood clotting, which can cause frequent nasal bleeding and prolonged bleeding of cuts, as well as increased production of the sebaceous gland and occasional acne. Some athletes also report headaches and sexual overstimulation. When very high dosages are taken over a prolonged period, spermatogenesis can be inhibited in men, i.e. the testes produce less testosterone. The reason is that Deca-Durabolin, like almost all steroids, inhibits the release of gonadotropins from the hypophysis.

Women usually experience no major problems with Deca Durabolin. At higher dosages androgenic-caused virilization symptoms can occur, including deep voice (irreversible), increased growth of body hair, acne, increased libido, and possibly clitorishypertrophy.