The birth control shot is called Depo Provera in the US. It is a shot given once per month and is very effective in preventing pregnancy. The side effects of the shot are irregular, unpredictable periods; weight gain; and decreased estrogen levels that can lead to bone mineral density loss. Most women do well on the shot and have few side effects.
The bleeding with the birth control shot can be hard to predict. Some women will have normal menses, but most will not. On average, there is less bleeding with the shot than with no birth control, but the bleeding is often erratic. Spotting is common. This can be anything from daily to rare spotting. Most women will eventually stop having any bleeding if they stay on the shot long enough.
Weight gain from the shot can be significant. The average weight gain is small and many women will not have any weight gain. A subset of the population will gain a lot of weight on the shot. I have seen women gain 70 pounds or more in a year. This type of weight gain is rare, but it can happen. It is hard to predict who will gain weight on the shot.
Depo Provera does not have any estrogen, so for women who are sensitive to the side effects of estrogen, this might be a good birth control option. The shot can inhibit estrogen production from the ovaries and can cause side effects related to decreased estrogen levels. Decreased bone mineral density is the side effect that causes the most concern.
Women who are on the birth control shot or are considering it may consider using the implant instead. The contraceptive implant is more reliable, lasts for 3 years, and does not cause weight gain, or decrease estrogen levels.