Weight gain is a common birth control pill side effect. Certain oral contraceptives are more likely than others to increase the pounds. Fortunately, the weight gain is often due to temporary water retention and will go away in a few months. Why do birth control pills make me gain weight? Water retention is a typical symptom of premenstrual syndrome that is caused by fluctuating hormone levels. You’ve probably noticed it before—about a week or two before your period starts you start to feel bloated and your clothes are a little too tight. Estrogen-containing birth control pills have the same effect. The most widely used type of birth control pill contains two types of synthetic female hormones—estrogen and progestin—that prevent your body from releasing an egg. It is the estrogen part of the pill that can cause you to retain water, which may make you feel like you’ve gained weight. Water retention usually passes after you’ve been taking the pill for a few months. Which types of pills are most likely to cause weight gain? There are two main types and many different brands of pills. They differ in the amount and types of hormones that they contain. The combination pill is more likely to cause you to gain weight. It contains both estrogen and progestin to prevent pregnancy. The estrogen part of the pill can make you feel like you’ve gained weight because it causes your body to retain more water than usual. A few common brands of the combination pill include Ortho Tri Cyclen, Brevicon, Loestrin, and Mircette, but there are several other brands available. The combination pill may contain the same level of estrogen and progestin in each active pill, or the amounts may change during the cycle. As a rule of thumb, the lower the level of estrogen in your pill, the less likely you are to gain weight. The mini pill contains progestin only. Since it does not contain estrogen, the mini pill is not likely to cause you to gain weight. Brands of the mini pill include Micronor and Jolivette. Women who are breastfeeding or who have medical conditions that prevent them from tolerating estrogen medications often take the mini pill. Estrogen has been linked to an increase in blood clots, stroke, and heart attack. What can I do to control water retention? While some women may have a harder time with water retention than others, you may find that a few lifestyle changes can limit the amount of water you retain. Try these tricks: Skip the salt. Salt also causes you to retain water so don’t add it to the foods you cook. When eating out, ask that your food be prepared without salt if possible. Take a walk. It’s true that a little aerobic exercise can help your metabolism and allow your body to remove the extra fluids that lead to weight gain. Ask for half-caff. Caffeine also makes you retain water. The next time you order that double latte, ask for half the caffeine—one shot regular, one shot decaf. You probably won’t miss the second shot of espresso and you’ll avoid some of the post-caffeine crash. If you gain a little weight when you start the pill, you’ll most likely drop back to your normal weight within a few months, after your body's metabolism adapts to the pill.