Megestrol acetate is a synthetic form of progesterone used primarily to stimulate appetite and promote weight gain in people with AIDS-related wasting, and to treat advanced breast and endometrial cancers. It comes in both tablet and liquid forms, with each formulation tied to specific uses.
FDA-Approved Uses
Megestrol has two distinct approved roles depending on the formulation. As an oral suspension (liquid), it is approved for treating anorexia, cachexia, or unexplained significant weight loss in people diagnosed with AIDS. Cachexia is severe, progressive muscle and weight loss that doesn’t respond to simply eating more. For people experiencing this kind of wasting, megestrol is one of the most potent appetite-stimulating medications available.
In tablet form, megestrol is approved for the palliative treatment of advanced breast cancer and advanced endometrial cancer. “Palliative” here means the goal is to slow disease progression and manage symptoms rather than cure the cancer. Because megestrol is a hormone therapy, it works by disrupting the hormonal balance that certain cancers depend on to grow. Some breast and endometrial tumors need estrogen or other hormones to fuel their growth, and megestrol interferes with that signaling.
How Megestrol Promotes Weight Gain
Megestrol triggers appetite through hormonal pathways, and it does this effectively. However, the type of weight it adds is almost entirely fat. In older adults and people with cancer or kidney failure, fat is the principal or only component of the weight gained. One study found that body weight increased significantly but skeletal muscle mass actually dropped by 5% within three months of use. This is an important distinction: megestrol can help you eat more and gain weight, but it won’t rebuild the muscle that wasting diseases break down.
That tradeoff matters when weighing the benefits. If the goal is to restore appetite and add fat mass during severe weight loss, megestrol is effective. If the goal is to regain strength or functional muscle, it falls short. Because of its properties that work against muscle building, megestrol is very unlikely to preserve or increase muscle mass during cachexia.
Off-Label Uses
Doctors sometimes prescribe megestrol for conditions beyond its official approvals. One well-studied off-label use is treating endometrial hyperplasia, a condition where the lining of the uterus grows abnormally thick. In a study of 52 postmenopausal women who were poor candidates for surgery and had precancerous changes in the uterine lining, continuous megestrol treatment produced complete remission in more than 90% of cases. Treatment lasted anywhere from 9 to over 100 months.
Megestrol has also been used off-label to manage cancer-related appetite loss beyond AIDS wasting, to treat hot flashes, and as a component of fertility-sparing treatment for early endometrial cancer in younger women who want to preserve the option of pregnancy.
Tablet vs. Liquid Forms
The two formulations aren’t interchangeable, and how you take megestrol matters for how well your body absorbs it. The standard oral suspension absorbs poorly on an empty stomach. A newer nanocrystal formulation was developed to address this problem, delivering peak blood levels nearly 7 times higher than the standard liquid when taken without food. When taken with a meal, both formulations perform similarly. If you’re prescribed the standard liquid form, taking it with food makes a significant difference in how much of the drug actually enters your bloodstream.
Blood Clot Risk
The most serious safety concern with megestrol is an increased risk of blood clots. The FDA labels carry a specific warning to use caution in anyone with a history of thromboembolic disease. A study of 179 patients with non-small cell lung cancer found that those taking megestrol developed deep vein thrombosis at 3.4 times the rate of patients who didn’t take it, after adjusting for gender. Even after adjusting for cancer stage, the rate was still 2.8 times higher. This risk is worth discussing with your doctor, particularly if you have other risk factors for blood clots like immobility, recent surgery, or a personal history of clotting disorders.
Common and Serious Side Effects
Many people tolerate megestrol without major issues, but side effects do occur across a range of severity. The more common ones include decreased sexual desire, impotence, unexpected vaginal bleeding, difficulty sleeping, gas, and rash.
More concerning side effects require immediate medical attention. These include:
- Signs of high blood sugar: extreme thirst, frequent urination, extreme hunger, blurred vision
- Signs of blood clots: leg pain, difficulty breathing, crushing chest pain, sudden weakness or numbness in an arm or leg, slow or difficult speech
- General warning signs: nausea, vomiting, dizziness, weakness
The blood sugar connection is particularly relevant for people with diabetes or prediabetes, since megestrol can raise glucose levels enough to cause symptoms even in people who haven’t had blood sugar problems before.
Who Should Not Take Megestrol
Megestrol is contraindicated in pregnancy. It can cause fetal harm, and women of childbearing age should use effective contraception while taking it. It’s also contraindicated in anyone with a known allergy to megestrol acetate or any inactive ingredient in the formulation. People with a history of blood clots aren’t necessarily excluded, but their doctors should weigh the risks carefully given the elevated clotting risk the drug carries.