The Depo-Provera shot is 99.8% effective at preventing pregnancy when used perfectly, meaning you get every injection exactly on time. In real-world use, that number drops to about 94%, which translates to roughly 6 out of 100 women becoming pregnant in the first year. The gap between those two numbers comes down to one thing: missed or late injections.
How the Shot Prevents Pregnancy
The shot works through three separate mechanisms, which is part of why it’s so reliable when used correctly. First, it stops your ovaries from releasing an egg each month. Without ovulation, there’s nothing for sperm to fertilize. Second, it thickens the mucus at the opening of the cervix, making it harder for sperm to get through. Third, it thins the uterine lining, which makes implantation less likely even if the first two defenses fail.
How It Compares to Other Methods
A large prospective study tracked pregnancy rates across different contraceptive methods and found that women using the shot had a failure rate of just 0.22 per 100 person-years, nearly identical to the 0.27 rate for IUDs and implants. By contrast, women on the pill, patch, or ring had a failure rate of 4.55 per 100 person-years, more than 20 times higher.
That study captured women who were closely followed and supported, so the numbers reflect near-ideal conditions. National survey data paints a more realistic picture: a 4% failure rate for the shot, compared to 1% for IUDs and implants and 7% for the pill. The shot lands in the middle. It’s significantly more forgiving than daily pills, but not quite as hands-off as a device that sits in your body for years.
Timing Your Injections
You need a new shot every 12 weeks (about 3 months). The CDC guidelines allow a 2-week grace period, so you can be up to 15 weeks from your last injection without needing backup contraception. If you’re more than 2 weeks late, you can still get the shot, but you’ll need to use condoms or abstain for the next 7 days while protection rebuilds.
If you get your first shot within the first 7 days of your period, you’re protected immediately. If you start at any other point in your cycle, most providers recommend using a backup method for the first 7 days.
Weight Gain: What the Data Shows
Weight gain is one of the most common concerns people have about the shot, and it does happen, though the amount varies widely from person to person. In two large clinical trials, the average weight gain after one year was modest: about 1.4 to 1.7 kilograms (roughly 3 to 4 pounds). After three years of continuous use, the median gain climbed to about 4.4 to 5.1 kilograms (10 to 11 pounds). But the wide range of individual results is notable. Some women gained significantly more, others gained nothing, and some lost weight.
Bone Density and Long-Term Use
The shot carries an FDA warning about bone mineral density loss. Women who use it can lose bone over time, and that loss increases the longer you stay on it. The FDA recommends against using the shot for longer than 2 years unless other methods aren’t a good fit for you. This is particularly relevant for teens and young adults, since that’s when your body is still building peak bone mass.
The reassuring part is that bone density does recover after stopping the shot, though not always completely. In adults, bone density at the hip and spine partially recovered within 24 months of the last injection. In adolescents, hip bone density hadn’t fully bounced back even after 5 years. Getting enough calcium and vitamin D while on the shot is a reasonable precaution, though no studies have confirmed it directly prevents bone loss from the injection.
How Long It Takes Fertility to Return
The shot is not a permanent form of birth control, but it does take longer to wear off than most other methods. Each injection lasts about 15 weeks, and after your last dose, the median time to conception is roughly 9 months from that final shot. That 9-month figure includes the tail end of the drug’s effect plus an additional 5.5 months for your cycle to normalize and conception to occur. If you’re planning a pregnancy in the near future, this delay is worth factoring in. Other methods like the pill or IUD allow fertility to return within weeks.
What Makes the Shot More or Less Effective
The single biggest factor in the shot’s effectiveness is staying on schedule. Unlike the pill, you don’t have to remember something every day. You only need to show up four times a year. But that infrequency can work against you: it’s easy to lose track of when your next appointment is due, especially if you don’t have a reminder system in place. Setting a calendar alert for 11 weeks after each injection gives you a buffer to schedule the next one before the grace period runs out.
Body weight does not reduce the shot’s effectiveness in the same way it can with some oral contraceptives. The dose is consistent regardless of size, and the hormone is released steadily from the injection site over the full 12-week period.