The birth control pill is an imperfect—though still awesome—invention. Even though it's been one of the most popular birth control methods for decades, many women have a love-hate relationship with their pill. On top of the fact that the basic idea hasn't changed since it first became a contraceptive option in the 1960s (three weeks of daily pills followed by an unnecessary placebo week—really?), side effects of the birth control pill range from annoying to truly problematic.

But not all versions of the pill affect women the same way. There are two main categories of birth control pills—combined hormonal pills, which contain doses of estrogen and progestin, and "mini pills," or progestin-only pills—and the type you're using matters. "Everyone’s body is different and can react differently to the pill depending on many factors—a big one being their own health history," says Janell Sanford, Pharm.D., pharmacist in charge at the Pill Club. In other words, you might simply tolerate one form of the pill better than another.

So how do you know when you need to switch? We asked the experts what's normal, which side effects of the pill you should never put up with, and when it's time to switch.

Side Effects of the Pill: What's Normal

Anytime you tinker with your body's hormonal balance, there are bound to be side effects. But the good news is, most of these are temporary and will go away after your body has a couple of months to adjust, says Sanford. "For example, although weight gain is not a confirmed side effect of birth control, some pills can cause temporary water retention, which feels like weight gain and usually resolves after one, two, or even three cycles," she says. Some of the most common side effects women report: spotting, mood changes, and breast tenderness.

Other side effects are more serious. "The major health consequence of estrogen-containing birth control pills is a slightly increased risk of blood clots," says Mary Jane Minkin, M.D., clinical professor of obstetrics, gynecology, and reproductive sciences at Yale University School of Medicine. If you notice chest or leg pains, swelling or redness in your legs, shortness of breath, eye problems, or blurry vision, call your doctor ASAP. Severe headaches or abdominal pain can also be a sign something more serious is going on, says Sanford. If you have a severe headache that doesn't go away with over-the-counter painkillers or abdominal pain that doesn't feel like your average cramp (and especially if it occurs on the upper right side of your stomach), let your doctor know.

Signs It's Time to Switch

If you notice any of the serious side effects above, talk to your doctor ASAP. For the more common—but still annoying—ills, here's how to talk to your doctor about finding a better fit.

If you're still spotting after a couple of months… Breakthrough bleeding is more likely to happen on lower-dose birth control pills. "Some women will develop spotting when they shouldn't be bleeding," says Dr. Minkin. "Switching it up to a slightly stronger pill [with a higher dose of hormones] might just be helpful."

If your libido is tanking… The pill also has the potential to affect your sex drive. If you've noticed your libido feels lower since starting a new brand of birth control, Dr. Minkin recommends talking to your doctor about switching to a progestin-only pill that uses levonorgestrel, which might be a better fit for your body.

If you're getting headaches… If you're getting low-grade headaches since starting a new pill, talk to your doctor about switching to a pill with a lower dose of estrogen, says Dr. Minkin.

"I always encourage women to keep notes about what side effects you experience on which pill," says Dr. Minkin. If your doc prescribes a new brand and you suddenly notice a change in your mood that wasn't present with your last pill, for example, tell her. Even if you've tried a few different pills and still don't feel you've found the right fit, these notes can be especially helpful as you and your doctor figure out the right type of birth control for you. "For someone who has never taken a pill before and is in good health, I would suggest starting with a pill with 20 micrograms of estrogen," says Dr. Minkin. "And always discuss any concerns about side effects with your provider, who can guide you."