Weight Loss Medications

Reviewed on 8/15/2022

What are weight loss drugs and how do they work?

Taking a weight loss drug may not result in weight loss by itself. But using diet pills can help an overweight person stay on a diet because nearly all of these medications work on suppressing appetite. "Feeling full" is believed to be related to a number of biochemical processes in the body. Signals to indicate fullness come from fat cells and the gastrointestinal tract; these converge with signals in the central nervous system.

Appetite suppressants target a couple of key neurotransmitters in this process: serotonin and norepinephrine. Increased levels of serotonin result in a feeling of fullness. Increasing norepinephrine levels stimulate the central nervous system, decreasing appetite.

  • Only one drug among the weight loss medications works in a different way. Orlistat (Xenical, Alli) works in the gastrointestinal tract to prevent the absorption of about a third of ingested fat.

What are the different types of weight loss drugs? 3 Types

Three types of drugs are used in weight loss therapy.

  • Stimulant-like drugs stimulate the central nervous system and reduce appetite.
  • Sibutramine (Meridia) increases levels of serotonin and norepinephrine, helping you feel full.
  • Orlistat prevents a sizable amount of fat absorption in the gut.

Regardless of how these weight loss pills work, they only help reduce weight if the user is also limiting calorie intake.


The Best Diet Tips: How to Lose Weight the Healthy Way See Slideshow

Who is a good candidate for weight loss drugs?

Weight loss medications are best suited for those with BMIs of 27 or greater when there is at least one other risk factor (such as diabetes or high cholesterol) present or in patients with no other risk factors who have BMIs of 30 or greater. However, these guidelines are not absolute.

Physicians may avoid using certain prescription diet pills in patients with hypertension, cardiac disease, hyperthyroidism, or glaucoma as well as in those with a history of drug abuse.

What are the side effects of weight loss drugs?

Side effects of the stimulant-like prescription diet pills include

Side effects of sibutramine include

For the diet pill orlistat, side effects include

  • diarrhea,
  • oily stools,
  • gas,
  • flatulence, and
  • a decrease in absorption of fat-soluble vitamins.

What are the warnings with weight loss drugs?

Before starting any medication, let your doctor know your full medical histories such as drug allergies, medical conditions, current medication use, and whether you are pregnant, trying to become pregnant, or nursing.

Amphetamine-Like Drugs

Because patients can develop tolerance to this type of prescription diet pill within weeks, they are indicated for short-term use.

Using this type of weight loss medication may also result in drug dependence and abuse. Side effects seen with chronic use include irritability, personality changes, insomnia, and even psychosis similar to schizophrenia. Sudden withdrawal may result in severe depression.

Operating a motor vehicle or machinery may be more difficult while using these weight loss drugs. People with heart disease, high blood pressure, thyroid problems, glaucoma, or epilepsy may not be able to take these medications.

Phentermine (Adipex-P, Fastin)

Pulmonary hypertension is a rare and potentially fatal blood vessel disease of the lungs that reportedly occurred when the amphetamine-like drug phentermine was used in combination with two now-withdrawn weight loss products: dexfenfluramine (Redux) and fenfluramine (Pondimin). Rarely, it has occurred in those taking phentermine alone. Symptoms include trouble breathing, chest pain, fainting, and edema.

Similarly, regurgitant cardiac valvular disease, or a leaky heart valve, has been linked to phentermine use in combination with dexfenfluramine and fenfluramine, but this effect possibly could occur from phentermine use alone.

People who are already taking medications or have medical conditions such as high blood pressure and diabetes need to talk to their doctor about the risks of taking phentermine.

Diethylpropion (Tenuate):

High doses may result in hallucinations; toxic psychosis is also possible with excessive or even proper use.

Diethylpropion may increase the risk of convulsions in epileptics.

Sibutramine (Meridia)

Sibutramine can substantially raise blood pressure and/or pulse rate in some patients. Because of this, sibutramine users should have their blood pressure and pulse rate monitored regularly. The drug should be used with caution, if at all, in patients with a previous history of hypertension, stroke, heart disease, or severe kidney or liver disease.

Patients with narrow-angle glaucoma should not use sibutramine because it causes dilation of the pupils (mydriasis). Sibutramine may interact with other medications, so it is important to talk to a doctor about current medications before starting sibutramine.

Orlistat (Xenical, Alli)

High-fat diets or meals may increase side effects from orlistat. Orlistat inhibits the absorption of fat-soluble vitamins. Patients should take a multivitamin containing fat-soluble vitamins with two hours' separation from the orlistat dose.

Orlistat may increase the risk of gallbladder and kidney stones. People taking medications for diabetes, thyroid disorders, or other medical problems should talk to their doctor about possible drug interactions before taking orlistat.


Weight loss occurs in the belly before anywhere else. See Answer

What are the drug interactions with weight loss drugs?

Phentermine may decrease the effectiveness of guanethidine.

Tenuate may interact with blood pressure medications, insulin, and other medications such as Thorazine.

Sibutramine should not be used with, or within two weeks of, monoamine oxidase inhibitors (MAOIs). Doing so may precipitate a serious and sometimes fatal side effect known as the "serotonin syndrome." Sibutramine can interact with other antidepressants, narcotic pain medication, antibiotics, and migraine medications.

Because sibutramine commonly raises blood pressure, patients should be carefully monitored for use of other drugs which can do the same such as over-the-counter cough, cold, and allergy preparations containing such ingredients as pseudoephedrine.

Orlistat may reduce levels of supplemental vitamins or medications such as cyclosporine. Patients taking both drugs should separate dosages by two hours.

What are some examples of weight loss drugs?

Here are some of the weight loss medications.

What are OTC diet pills?

Only one over-the-counter diet pill is approved for weight loss: a reduced-dose formulation of orlistat known as Alli.

  • It works by preventing the absorption of about a third of ingested fat.
  • While side effects from Alli are slightly lower than with its prescription cousin (because of the lower dose): they remain unpleasant: oily stools and discharge and potential bowel accidents if too much fat is eaten in one sitting.

Alli's manufacturer recommends keeping total fat consumption to about 30% of calories a day and spreading it out over three meals. The net weight loss effect with Alli: is about 50% greater than diet and exercise alone.

What do I need to know about OTC diet supplements?

Many other over-the-counter diet supplements are promoted as helping with weight loss, but few have actually been proven to work.

  • Worse, some of the ingredients used in OTC diet supplements may be dangerous.
  • The Food and Drug Administration classifies herbal products as dietary supplements. This means that they are unregulated and can be marketed without the years of testing and regulatory review required for prescription (and nonprescription) drugs.

Remember that supplements can have side effects, and you should check with your doctor first before taking any type of supplement, including OTC diet pills.

Here are some common ingredients seen in OTC diet supplements:

Green tea extract: cases of liver problems in people using concentrated green tea extracts have been reported.

Hydroxycitric acid: Derived from the fruit of a tree native to Southeast Asia. It generally appears to be safe, although one maker recalled its products after they were associated with liver damage. Other health problems reported included seizures, cardiovascular disorders, and serious muscle damage. These products contained many other ingredients, and it is unclear which ingredients or doses were associated with liver problems.

Chromium: A mineral that people can get through diet, particularly meats, whole grains, and some vegetables and fruits. It has been linked to side effects such as headaches and dizziness as well as more serious health problems at high doses.

Conjugated linoleic acid (CLA): Found naturally in meat and dairy products, it may cause stomach upset.

Hoodia: Derived from an African plant native to the Kalahari Desert. Hoodia products typically contain other additional ingredients. Its safety is not yet known.

Chitosan: Made from the starch found in shellfish.

Pyruvate: Produced by the body as a result of the breakdown of carbohydrates and protein from food and found naturally in foods such as cheese, wine, and red apples.

St. John's wort: Used mainly as an antidepressant, this herb can interact with numerous other drugs.

Aloe: Sometimes marketed as an "internal" cleanser, aloe causes a strong cathartic effect in the intestines. That can lead to mineral depletion or worse if users have pre-existing intestinal issues, such as ulcerative colitis.

Cascara: An effective laxative but ineffective weight loss agent, cascara interacts with other drugs and can throw off the body's mineral balance.

Glucomannan: Derived from a plant root, glucomannan has been banned in several countries because when exposed to liquid it swells and can result in gastrointestinal obstruction.

Guarana: A natural stimulant, guarana can increase blood pressure.

Yerba mate: Often used in tea, yerba mate ingestion can result in high blood pressure and overstimulation of the central nervous system. It may also be linked to esophageal cancer.

Guar gum: It is used in the food and pharmaceutical industries as a thickening agent but taken alone, guar gum can swell on contact with liquid, potentially leading to an obstruction.

Ephedra (ma huang): Consumers should not be able to find OTC supplements containing this ingredient because the FDA banned its sale in dietary supplements in 2004. The use of ephedra can cause high blood pressure and other cardiovascular problems.

www.healthyweightforum.org (Healthy Weight Forum)
Nutr Metab Cardiovasc Dis 2008 Feb; 18(2): 158-68 (website: www.ncbi.nih.nlm.gov)
www.teachersdomain.org (Teachers Domain)
www.obesityaction.org (Obesity Action Council)
www.dailymed.nlm.nih.gov (Daily Med)
www.aarp.org (American Association of Retired Persons)
NIH, Office of Dietary Supplements website
Over-the-Counter and Herbal Remedies for Weight Loss https://www.webmd.com/diet/guide/herbal-remedies


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