What are the possible side effects of DMPA? DMPA has few side effects. However, side effects may include: Small weight gain in some women Headaches Acne Change in sexual interest Mood changes The injection is long acting and if side effects occur they may last up to 3 months (it is not possible to reverse the effects of an injection once it is given). Some side effects (especially bleeding changes) may persist beyond 3 months although there are a number of treatment options available with varying success that may help reduce bleeding problems if they occur.