Depo-Provera is 99.8% effective when used perfectly, meaning you get every injection exactly on schedule. In real-world use, that number drops to about 94%, with roughly 6 out of 100 people becoming pregnant in the first year. The gap between those two numbers comes down to one thing: getting the shot on time.
Perfect Use vs. Typical Use
The distinction between perfect and typical use matters more for the shot than for many other contraceptives. With perfect use (every injection on schedule, no exceptions), only 0.2% of users become pregnant in a year. That’s comparable to an IUD. But with typical use, the failure rate jumps to 6%, which is significantly higher than methods that don’t require you to remember appointments.
The entire effectiveness of the shot depends on returning every 13 weeks. If you go beyond that 13-week window, you may have already started ovulating again, and your protection drops. There’s no built-in grace period. If you’re late, a pregnancy test is needed before you can get the next injection. This strict schedule is the single biggest reason the typical-use failure rate is 30 times higher than the perfect-use rate.
How the Shot Prevents Pregnancy
The injection works in two ways. First, it stops your ovaries from releasing an egg, so ovulation simply doesn’t happen. Second, it thickens the mucus at the opening of the uterus, creating a barrier that makes it difficult for sperm to pass through. Within 24 hours of the injection, cervical mucus becomes unfavorable for sperm penetration in about 90% of women. That fast-acting cervical effect is part of why the shot works so well when you stay on schedule.
How It Compares to Other Methods
In perfect use, the shot is in the same league as IUDs and implants, which are all close to 99% or higher. The difference shows up in typical use. IUDs and implants stay close to 99% effectiveness in real-world conditions because once they’re placed, there’s nothing to remember. The shot, by contrast, requires an office visit four times a year, and missed or late appointments are common enough to push the failure rate to 6%.
If you’re comparing options, the question is really about your own scheduling habits. If you’re confident you can keep every appointment on time, the shot performs nearly as well as a set-it-and-forget-it method. If keeping a strict quarterly schedule feels unreliable for your life, a long-acting method will give you better real-world protection.
When Protection Starts
Timing your first injection matters. If you get the shot within the first seven days of your menstrual cycle, it reliably suppresses ovulation right away. Research shows that injections given after day 7 allowed ovulation to occur in some women, so if you start later in your cycle, you’ll need a backup method like condoms for at least seven days. Your provider can help you figure out the right timing based on where you are in your cycle.
Medications That Can Reduce Effectiveness
Certain drugs speed up the breakdown of hormones in your body, which can lower the shot’s effectiveness. The FDA lists several, including some seizure medications (like carbamazepine, phenytoin, and topiramate), the antibiotic rifampin, the antifungal griseofulvin, and the herbal supplement St. John’s wort. If you take any of these, you may need a backup contraceptive method or a different birth control approach altogether. The effect of St. John’s wort varies depending on the specific product and dose, making it particularly unpredictable.
Return to Fertility After Stopping
One important consideration with the shot is that fertility doesn’t return immediately. Unlike the pill or an IUD, where ovulation can resume within weeks, the hormones from the injection clear your system gradually. Most people see their fertility return within several months after the last shot, but for some it can take 10 months or longer before ovulation resumes on a regular basis. If you’re planning a pregnancy in the near future, this delay is worth factoring into your decision. It doesn’t affect long-term fertility, but it does mean you can’t simply stop the shot and expect to conceive right away.
Making the Shot Work at Its Best
The shot’s effectiveness is almost entirely in your hands. Set a recurring reminder for every 12 weeks (not 13, to give yourself a buffer). Some clinics offer text or email reminders for upcoming appointments. If you know you’ll be traveling or busy around your next injection date, schedule it a week early rather than risk being late. There’s no harm in getting the shot a few days ahead of schedule, but getting it even a few days late can open a window for pregnancy.
At its best, the shot is a highly effective, low-maintenance contraceptive that requires attention only four times a year. At its worst, missed appointments turn a 99.8% method into a 94% one. The method itself is excellent. The challenge is the schedule.