Prescription Drugs Information

Diet Pills

Appetite Suppressant Drugs


About diet pills

Diet pills are drugs taken to control weight. They usually work as appetite suppressants – decreasing hunger or making the body feel full more quickly than usual after eating. Some diet pills may increase the body’s metabolism to burn more fat, a process known as thermogenesis.

Weight control is an important aspect of preventing and managing numerous health conditions, such as diabetes, high blood pressure and heart conditions. However, diet pills should be used only under the strict supervision of a physician and in conjunction with a healthy diet and exercise plan.

Most diet pills approved by the Food and Drug Administration (FDA) are effective only for short–term use, such as a few weeks. Weight loss of 10 to 15 percent is considered a good response to the drugs. Patients may be disappointed if they believe that these drugs can remove unwanted fat and keep it off forever. The amount of weight lost while taking these drugs is usually less than people hope for, although some patients have lost more than 20 pounds.

Studies have shown that most people experience modest weight loss (sometimes less than 5 pounds) for the first six months after beginning treatment to lose weight. After the first six months, many patients stop losing weight. Unless patients have developed a healthy pattern of eating and exercise, they can expect to regain weight lost while taking diet pills. Even when long-term use of diet pills is approved by a physician, it must be accompanied by a healthy diet and exercise to reap the full benefits of such treatment.

The FDA has approved only two diet pills for long-term treatment of obesity: sibutramine (Meridia) and orlistat (Xenical). Sibutramine is an appetite suppressant. Orlistat is a lipase inhibitor and works in a different way than sibutramine and other appetite suppressants. Lipase inhibitors reduce the amount of fat that can be broken down in the body, making it harder for the body to absorb the fat. In 2007, a lower-dose version of orlistat (Alli) became the only over-the-counter diet pill approved by the FDA.

In addition to FDA-approved diet pills, there are a wide variety of over-the-counter and herbal supplements that are marketed as a treatment for obesity. Some of these drugs even claim to promote weight loss without the patient having to make any lifestyle changes.

Although some of these products may result in short-term weight loss, there is no evidence that any non-FDA-approved drugs can produce significant, long-term weight reduction. People generally regain weight they lost when they stop using the pills, and this “yo-yo effect” has been shown to promote weight gain over time. Some over-the-counter diet pills have also been associated with a number of serious health risks and may be especially dangerous for people with certain conditions, including diabetes, thyroid disease or cardiovascular problems.

Many diet pills, including those that are approved by the FDA, may have side effects that are dangerous for people with diabetes or other disorders. Some diet pills (e.g., ephedra, fenfluramine) have been taken off the market after studies showed that they caused life-threatening health problems. It is important that patients consult their physician before taking any type of medication or supplements.

The FDA warns against using hormonal thyroid medications (a treatment for hypothyroidism) in an attempt to treat obesity in people with normal thyroid function. High doses of these medications may cause serious or even life-threatening toxicity, especially if combined with amphetamines.

The FDA has also warned against purported weight-loss supplements that actually contain prescription medications or other potent substances, such as:

  • Strong thyroid hormones that could cause heart attack, stroke or other serious conditions
  • A potentially deadly combination of stimulants, hormones, tranquilizers and laxatives
  • Antidepressants or other psychiatric medications

Some medications, including diuretics, laxatives and emetics (vomit inducers), and illegal drugs are also sometimes abused in an attempt to lose weight. Such misuse can have dangerous consequences.

A new class of injected antidiabetic agents known as incretin mimetics has promoted weight loss in many patients and sparked interest from nondiabetics. However, it is approved only to treat type 2 diabetes and is not to be used as a weight-loss aid in nondiabetics. Metformin, which is in the biguanide class of antidiabetic agents, has helped some patients lose weight by promoting a feeling of fullness after meals. It is sometimes prescribed off-label for prediabetes, insulin resistance or polycystic ovarian syndrome, conditions that typically involve excess weight.


Types and differences of diet pills

Many types of diet pills claim to treat obesity and help people to lose weight. Some have been approved by the Food and Drug Administration (FDA) to treat obesity and are available only by prescription. Others are available over the counter and may contain chemicals or herbal ingredients that might or might not be effective.

Most diet pills are classified as appetite suppressants. They work by altering the levels of two chemicals in the brain called serotonin and catecholamine. These chemicals are associated with numerous aspects of mood and behavior, including appetite. By increasing the levels of serotonin and catecholamine, the body is tricked into thinking that it is not hungry. However, regular exercise and a healthy diet must accompany any type of weight-loss treatment plan in order to maintain weight loss.

Appetite suppressants that have been approved by the FDA to treat obesity include:

  • Diethylpropion (Tenuate, Tenuate dospan)
  • Mazindol (Sanorex, Maznor)
  • Phendimetrazine (Brontril, Plegine, Prelu-2, X-Trozine)
  • Phentermine (Adipex-P, Fastin, Ionamin, Oby-trim, Pro-Fast, Zantryl)
  • Sibutramine (Meridia)

Another FDA-approved medication for obesity is orlistat (Xenical). This type of diet pill is a lipase inhibitor, which works differently than appetite suppressants. Rather than reducing hunger, lipase inhibitors block the action of an enzyme called lipase. Lipase is responsible for breaking down fat in the intestines so it can be easily absorbed by the body. If this action is blocked, the body cannot efficiently absorb the fat, reducing the amount of calories derived from food. The fat is instead excreted in the feces. Orlistat is available by prescription or in a lower dose without a prescription (Alli).

In addition to the diet pills that have been tested and approved by the FDA, there are a wide variety of over-the-counter drugs that claim to treat obesity. Some over-the-counter diet pills may be unsafe, especially for people who have additional medical conditions (e.g., diabetes, high blood pressure) or who are taking other medications. Some of the most common over-the-counter diet pills and ingredients include:


Ephedrine (or Ephedra) has been sold as both an energy enhancer and a “fat burner.” Studies have shown that ephedrine does indeed increase the body’s ability to burn fat and may result in short-term weight loss. However, recent studies have shown that ephedrine has toxic effects with short-term and long-term use. Controversy mounted in 2003 after the sudden death of a professional baseball player who was taking the drug. In 2004, the FDA banned the sale of products containing ephedrine, although it may be legally sold in tea. The herb ma huang is a source of ephedrine.

Chromium picolinate

A number of people take this substance to treat obesity and diabetes, though it has not been approved by the FDA for either purpose. The FDA does recommend that people consume 120 micrograms of chromium daily, a tiny amount, because scientists found that this mineral promotes the action of insulin in transporting glucose (blood sugar) to fuel the body’s cells. Some studies have suggested that chromium may help control glucose.

The combination of chromium with picolinic acid promotes the body’s absorption of chromium. Research has produced mixed results on whether chromium picolinate can promote weight loss. It has been linked to kidney problems, gene mutations and greater risk factors for developing cancer.


Carnitine is a nutrient found mostly in red meat and dairy products. It helps the body convert fatty acids to energy. Supplements such as L-cartinine (levocarnitine) have been touted as a weight-loss aid, but scientific research has not proven these claims. However, scientists are studying whether L-carnitine might benefit some patients, including people receiving hemodialysis for kidney disease and people with heart disease, hyperthyroidism, dementia, male infertility or some other conditions.


This African desert plant has been touted as an appetite suppressant. However, even if it is effective, some products claiming to contain hoodia have been found to contain very little or none of the herb.

Bitter orange

Also known as citrus aurantium, bitter orange is an herbal supplement derived from a type of orange. It contains chemicals that are similar to the banned substance ephedrine, and use may result in side effects including high blood pressure and arrhythmia (irregular heartbeat).


The body’s metabolism of glucose produces this substance. Limited research has suggested that large doses might help promote modest weight loss, but side effects may include diarrhea and upset stomach.


Chitosan is a chemical compound that is claimed by some to block the absorption of fat by the body. It is primarily used in agriculture as a plant growth enhancer. Although chitosan is generally safe to use, studies have shown that it is not effective as a diet pill and may result in the malabsorption of certain vitamins and minerals.

Green tea

This supplement, taken as a pill or as a tea, has been shown to slightly increase the body’s ability to burn fat and may result in modest, short-term weight loss. Green tea may provide other health benefits because it contains antioxidants. However, green tea may negatively interact with some medications.

Salicin and white willow

These ingredients are related to aspirin, which may have negative effects in some patients, such as those with diabetic nephropathy or other kidney disease.

Diet pills that have been withdrawn because of health concerns include dexfenfluramine (Redux), phenylpropanolamine (PPA), and the combination of fenfluramine and phentermine (fen-phen). The FDA has also banned supplements containing certain ingredients, including Aristolochia plant species, which are toxic to the kidneys, and guar gum, which in some cases has blocked the digestive tract.

Many over-the-counter diet pills contain stimulants (e.g., caffeine, ginseng, guarana) that are designed to speed up the body’s metabolism and aid weight loss. However, these products often contain large doses of stimulant that may result in adverse side effects, such as irritability, sleeplessness, high blood pressure and cardiovascular problems. Additionally, some research has shown that caffeine may be especially dangerous for people with diabetes because it can aggravate hypoglycemia and has been linked to insulin resistance. However, other studies have found that caffeine may help prevent diabetes or control glucose.


Conditions treated with diet pills

Diet pills are generally prescribed to treat obesity and should be used only under the strict supervision of a physician. For adults, obesity is defined as having a body mass index (BMI) above 30. However, the National Institutes of Health recommend that overweight adults with a BMI over 27 who also have obesity-related conditions (e.g., high blood pressure, type 2 diabetes) may benefit from diet pills. Diet pills should always be used in conjunction with a healthy diet and regular exercise.

Diet pills are not used to treat diabetes. However, weight control is an important aspect of managing diabetes and prediabetes. Diet pills may be recommended by a physician to reduce complications that may arise in people who have prediabetes or diabetes and are also overweight. Some of these complications may include:

  • Hyperglycemia
  • High blood pressure
  • High cholesterol
  • Heart disease
  • Insulin resistance, which in people with type 1 diabetes can cause double diabetes
  • Hyperinsulinemia
  • Metabolic syndrome

Conditions of concern with diet pills

Patients, especially those with diabetes, should consult their physician before taking diet pills, whether the drugs are prescribed or over-the-counter. Diet pills can affect the amount of insulin required by the body, which may result in higher dosages being needed. They may also affect glucose levels and cover up symptoms of hypoglycemia, which is of particular concern for people who have hypoglycemia unawareness (difficulty sensing low blood sugar).

In addition, diet pills may exacerbate many of the conditions that people with diabetes or other endocrine disorders are at risk for. These may include:

  • High blood pressure
  • Cardiovascular disease or heart attack
  • Stroke and transient ischemic attack (mini-stroke)
  • Kidney or liver problems
  • Thyroid conditions, including hypothyroidism and hyperthyroidism
  • Glaucoma

Additional conditions of concern with diet pills include:

  • Depression, bipolar disorder or other emotional disorders
  • Abuse of alcohol or drugs
  • Seizure disorders
Side effects

Potential side effects of diet pills

Common side effects that may be experienced when taking diet pills (both prescription and over-the-counter) include:

  • Nervousness or irritability
  • Trembling or restlessness
  • Elevated blood pressure
  • Trouble sleeping
  • Constipation, diarrhea or stomach cramps
  • Urinary tract disorders
  • Nausea or vomiting
  • Dry mouth
  • Headache
  • Heart palpitations

Orlistat, the lipase inhibitor (which inhibits the breakdown of fat in the body) approved by the Food and Drug Administration, decreases the absorption of fat-soluble vitamins (A, D, E and K). Physicians may recommend that patients taking orlistat also take certain vitamin supplements. Additionally, users of orlistat may experience side effects such as:

  • More frequent bowel movements
  • Inability to hold bowel movement
  • Cramping
  • Oily discharge in feces

Less common or rare side effects of diet pills include:

  • Mental disturbances (e.g., paranoia, hallucinations, delusions, depression, mania)
  • Numbness, particularly occurring on only one side of the body
  • Skin rash, hives or unusual bruising
  • Cancer

One of the major side effects of many types of diet pills is dependence and addiction. Users should be aware of the signs of dependence and discuss them with a physician if they occur. Signs and symptoms that may indicate dependence on diet pills include:

  • Strong desire for the medication
  • Dosage needing to be increased to obtain the same results
  • Withdrawal symptoms (e.g., stomach pain, shaking, fatigue, nausea or vomiting, depression) occurring after stopping the drug

Drug or other interactions with diet pills

Patients should consult their physicians before taking any additional prescriptions, over-the-counter medications, nutritional supplements or herbal medications. When taking diet pills, patients should not take other types of diet pills or weight-loss supplements unless approved by their physician.

Of particular concern to individuals taking diet pills are:

  • Stimulants. These include caffeine, amphetamines, cocaine and a number of prescription and over-the-counter diet pills. Medications that treat attention-deficit disorder are also stimulants. Caffeine, in particular, may interfere with glucose control and may make the side effects of some diet medications worse, especially in patients with diabetes.
  • Antidepressants. Antidepressants may be prescribed to treat depression or painful conditions including peripheral neuropathy. High blood pressure and an irregular heartbeat may result from the use of antidepressants and diet pills.
  • Thyroid medications. Drugs such as thyroid hormone may interact with diet pills. In addition, some over-the-counter substances marketed as diet pills have been found to contain dangerous levels of thyroid hormones.
  • Bronchodilators. These medications are prescribed in the treatment of asthma and other respiratory disorders.
  • Certain medications used during surgery, emergency medical treatments or dental procedures. Patients are encouraged to inform healthcare professionals if they are taking diet pills.
  • Over-the-counter medications to treat colds, hay fever, allergies, sinus problems and other minor health conditions.

Symptoms of diet pill overdose

  • High or low blood pressure
  • Irregular heartbeat
  • Severe stomach cramps or diarrhea
  • Confusion
  • Dizziness or fainting
  • Fast breathing
  • Panic
  • Hallucinations
  • Hostility or irritability
  • Nausea or vomiting
  • Restlessness
  • Shaking
  • Weakness or fatigue
How to use

Pregnancy use issues with diet pills

Pregnant or breastfeeding women are advised not to take diet pills. When taken during pregnancy, these drugs could cause withdrawal symptoms or even birth defects when taken in large doses. Some diet pills can also produce harmful effects in a child by passing through breast milk.

In general, women who are pregnant or nursing should consult a physician before taking any prescription or over-the-counter medication.

Child use issues

In general, physicians do not recommend diet pills for use in children. Despite the significant problems associated with childhood obesity, these drugs have not yet been approved for children under the age of 18.

Elderly use issues

Researchers have not yet examined the effects of diet pills on the elderly. Older patients may metabolize substances such as diet pills differently from younger adults and may be more likely to experience side effects.

Questions for your doctor

Patients may wish to ask their doctor the following questions about diet pills:
  1. Do I need diet pills to help me lose weight? Can I lose the weight I need to with exercise and a healthy diet?
  2. Do you think diet pills will help me control my diabetes or prediabetes?
  3. Should I take an over-the-counter or prescription form of diet pill?
  4. Are there any diet pills I should avoid?
  5. What type of diet and exercise do you recommend for me? Are there any other lifestyle changes you’d recommend for me?
  6. Should I consult a nutritionist? Can you recommend one?
  7. Could any of my medications interact with my diet pills?
  8. For how long should I take the diet pills?
  9. What are the potential side effects for this type of diet pill? At what point should I notify you of side effects?
  10. Are there any supplements or herbs I should avoid when using this pill?
  11. Could diet pills be dangerous for me if I am pregnant?

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