How quickly is the weight gain side effect of birth control occur?

Most don't gain: Most pills or other methods of birth control don't cause weight gain for most women. Placebo-controlled trials (comparing women taking sugar pills with those taking bc pills) have shown that both groups of women gain weight, but that women taking bc pills didn't gain more than those who were taking inactive/sugar pills. Depo provera, (medroxyprogesterone) bc shot is more likely to cause wt gain.