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Prescription vs. OTC Weight Loss Pills: Side Effects

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If you want to lose weight and are concerned with weight loss pills' side effects, use this guide to learn the side effects of each diet pill.

Prescription Diet Pills

Two types of diet pills exist: appetite suppressants and fat absorption inhibitors. They help you lose weight differently, causing different side effects.

Appetite suppressants either stimulate the nervous system or increase the levels of certain chemicals in your body. Side effects of stimulants include:

  • Pulmonary hypertension
  • Valvular disease
  • Increased blood pressure, pulse and heart rate
  • Restlessness
  • Dizziness
  • Insomnia
  • Dry mouth
  • Constipation
  • Nausea
  • Vomiting
  • Inability to operate motor vehicles and heavy machinery

All appetite suppressors include the following side effects:

  • Sweating
  • Thirst
  • Lightheadedness
  • Drowsiness
  • Stuffy nose
  • Anxiety

Fat absorption inhibitors prevent your body from breaking down and absorbing fat, releasing it through bowel movements. Side effects are stomach or intestinal related.

Meridia (Sibutramine)

This appetite suppressant increases your levels of serotonin and catecholamine. In addition to above, side effects include:

  • Headache
  • Anorexia
  • Insomnia
  • Sore throat
  • Dilated pupils in patients with narrow angle glaucoma

Xenical (Orlistat)

Xenical is a fat absorption inhibitor with the following side effects (many can be lessened with low fat dieting and taking a multivitamin 2 hours prior):

  • Abdominal cramps
  • Gas
  • Leaking oily stool
  • Increased number of and inability to control bowel movements
  • Diarrhea
  • Bloating
  • Risk of gallbladder and kidney stones
  • Reduces the body's ability to absorb vitamins

OTC Diet Pills

The FDA classifies OTC diet pills as herbal products or dietary supplements. Since they are unregulated, they may not work as advertised.


This appetite suppressant causes stomach problems.

Green Tea Extract

This appetite suppressant claims to increase fat and calorie metabolism, speeding up ability to lose weight. Side effects include:

  • Stomach problems
  • Vomiting
  • Bloating
  • Indigestion
  • Diarrhea
  • Liver problems


This diet pill claims to reduce body fat while building muscle. Side effects include:

  • Headaches
  • Dizziness

Conjugated Linoleic Acid

This appetite suppressant claims to reduce body fat and build muscle. Side effects include:

  • Upset stomach
  • Diarrhea
  • Indigestion

Guar Gum

This is both an appetite suppressant and a fat absorbtion inhibitor. Side effects include:

  • Diarrhea
  • Flatulence
  • Intestinal obstruction.


This fat absorption inhibitor causes constipation and bloating.


Alli is reduced strength Xenical and shares its side effects.

Weight Loss Pills to Avoid

These are dangerous diet pills. Some are banned, while others have dangerous side effects.

  • Yerba Mate
  • Glucomannan
  • Hydroxycitric Acid
  • Bitter Orange
  • Ephedra

Side effects of these include:

  • Increased blood pressure and heart rate
  • Over stimulation of nervous system
  • Esophageal cancer
  • Gastrointestinal obstruction
  • Muscle damage
  • Cardiovascular disorders
  • Seizures
  • Liver damage

Remember, diet pills may cause undesirable side effects or interference with other medication. Talk to your doctor before taking any medication to determine if it is right for you.

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