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