Cortisol is a hormone made by the adrenal glands. In addition to producing the “fight or flight” sensation you feel when you’re under stress, cortisol has the important function of reducing inflammation in the body.

Corticosteroids (often just called “steroids”) are synthetic versions of cortisol and are used to treat inflammatory conditions such as:

Corticosteroids are different from anabolic steroids that help build muscle.

According to research published in International Journal of Medical Sciences, about 40 million steroid prescriptions are written each year in the United States. Commonly prescribed steroids include:

These drugs are highly effective at reducing inflammation, but they also have some troubling side effects. One of these is weight gain. Read on to learn why this is the case and what you can do.

Many conditions that cause inflammation are due to a faulty immune system. Your immune system helps protect you from infection by recognizing things like viruses and bacteria as foreign bodies and mounting a chemical campaign to destroy them.

For reasons that aren’t always entirely clear, some people have immune systems that attack normal, healthy cells. This can result in damage and swelling to the body’s tissues. Steroids help fight that damage and swelling by reducing the chemicals that cause inflammation. They also help to suppress the immune system, so healthy cells aren’t attacked.

But steroids have some negative side effects, including weight gain. According to one study, weight gain was the most commonly reported adverse effect of steroid use, affecting 70 percent of those prescribed the drugs.

Steroids cause weight gain by altering the body’s electrolyte and water balances, as well as its metabolism — the way it uses and stores lipids, amino acids, protein, carbohydrates, and glucose, among other things. These factors contribute to weight gain by causing:

Many people on steroids notice increased fat in the abdomen, face, and neck. Even if you successfully control steroid-induced weight gain, you’re apt to look heavier while on these drugs because of this fat redistribution.

How much and even if you will gain weight (it’s not a definite) depends a lot of factors, including dose and duration.

Generally, the higher the dose of the steroid and the longer you’re on it, the more likely you are to encounter weight gain. Short courses of a few days to a couple of weeks don’t usually produce many side effects.

But one study published in the journal Arthritis Care and Research found that subjects who were on more than 7.5 milligrams of prednisone per day for more than 60 days were more likely to experience adverse side effects like weight gain than those on a lower dose for a shorter period of time.

The good news is, once the steroids are stopped and your body readjusts, the weight generally comes off. This usually happens within 6 months to a year.

The first step is talking to your doctor. Depending on the drug you’re taking and the disorder it’s treating, you may have other medication options.

Your doctor may also recommend a different dosing schedule or a different form of the steroid. For example, they may recommend every-other-day dosing or, if you have something like asthma, using an inhaled steroid that targets the lungs directly instead of a pill that can have full-body effects.

Don’t stop taking your medication (or changing when and how you take it) without medical guidance. Steroids are potent drugs that need to be tapered gradually. Stopping them abruptly can lead to serious health complications such as muscle stiffness, joint pain, and fever, not to mention a relapse of whatever disorder they were controlling.

To curb weight gain, use the same strategies you’d use to control weight in general:

  • Choose belly-filling (yet low-calorie) foods like fresh fruits and vegetables.
  • Stave off hunger by eating six small meals a day versus three large ones.
  • Choose fiber-rich and slower-to-digest complex carbohydrates versus refined ones (for example, whole wheat pasta instead of regular pasta, and brown rice instead of white).
  • Include a source of protein with each meal (meat, cheese, legumes, etc.). Research published in the American Journal of Clinical Nutrition found that meals that contain 25–30 grams of protein are most effective at curbing appetite and controlling weight.
  • Drink water. Besides filling you up, it can actually burn calories. One study published in the International Journal of Obesity found that overweight children who drank just 10 milliliters per kilogram of body weight of cold water increased their resting energy expenditure by 25 percent for 40-plus minutes after drinking.
  • Stay active. This is sometimes hard to do when you don’t feel well. Having a workout buddy can help, as can choosing an activity you enjoy.

Steroids are tremendously effective at treating some inflammatory conditions. But the drugs are potent and can produce some serious and unwanted side effects, such as weight gain.

If you’re on steroids and are worried about gaining weight, talk to your doctor about reducing your risk. In many cases, any weight gained during treatment will come off once the drugs are stopped, but that weight loss can take months to a year. Trying to prevent the weight gain before it becomes a problem is your best strategy.