If you do not start on the first day of your period, you need to use barrier protection such as a condom for at least two weeks to prevent pregnancy. Again, you should always use a condom to help protect against STDs. This shot is given every three months. This is a nice choice because you don't have to think about taking it everyday. Other side effects include weight gain, headaches, bloating, depression, and mood changes.
Many people have no side effects.
If you can't remember to take a daily pill, then you should consider the shot. These prevent pregnancy for up to 5 years. They are placed under your skin by a doctor. No one can tell that you have one. They are expensive.