Usually you're diligent about making sure you get your birth control pills refilled on time, but let's say you forgot and are overdue on a gynecologist exam. You don't have your new pill pack, but you do have a few questionably old boxes of birth control from when you briefly went off of the pill. Is it better to skip your daily pill until you get your script refilled, or can you use your old ones? It's complicated, because technically birth control pills do expire.

All medications, prescription and those sold over the counter, have set expiration dates, which are the manufacturer's way of telling you that they know a medication is both potent and safe until that date, says Katharine O'Connell White, MD, MPH, director, Fellowship in Family Planning, department of Ob/Gyn at Boston University, Boston Medical Center. Technically, after the expiration date, the medications may become less potent.

Most birth control pills tend to expire after one to five years, but there's a catch: "We know that most medications are still effective after their expiration date," Dr. White says. Studies from the Department of Defense have shown that stockpiled medications last months to years past the listed date. "It's not like the day after the expiration date it all of a sudden dissolves into dust and loses all its potency," she says. So, that's great news for those of us who may blank on getting our pills refilled, but that doesn't mean you can just ignore expiration dates altogether.

If you take a Tylenol that's expired, for example, you're not going to get sick, but it may not make your headache go away. "What you're risking is that it's going to work a little less well than you want it to," Dr. White says. But with birth control pills, if their potency goes down, they may not suppress ovulation. "You really don't want to risk that the pills are any less effective than they are," she says. Especially given how low the hormone doses already are for birth control pills nowadays, it's probably not worth the risk.

Interestingly, Dr. White says she often hears patients with IUDs complain that they think their IUD is "running out," because their periods come back. "The level of hormones in these devices does go down over time — but that doesn't mean that the device is running out and not going to work for you," she says. In fact, studies have confirmed that IUDs last longer than the FDA recommendation. So, even though you may see changes in your body over time, there are definitely enough hormones to last years beyond what it's labeled for.

Back to birth control pills, if your pills run out and you can't get a refill on time, Dr. White says you can use expired pills as a bridge to getting a new pack. "It may be better than nothing, but when you have the ability to get a fresh pack, that's always the way to go," she says. Definitely call your Ob/Gyn and make an appointment for ASAP, plus let them know that you have the expired pills. And, if you are having sex during this time and don't want to get pregnant, then it's wise to use another barrier method, like condoms.

In the future, if you have multiple packs, it's a good idea to just throw them all away except for one "in case of emergency" pack, Dr. White says. "You don't want to be tempted to count on them for your contraception."