What Does Phentermine Do to the Body?

Phentermine is an anorectic drug that is commonly used for weight loss. Prescribed by doctors to treat patients who have complications caused by obesity, such as diabetes, high blood pressure or high cholesterol, phentermine is usually prescribed for a short time -- generally three to six weeks -- to assist in weight loss. Individual reactions to the medication can vary, so usage of phentermine should be closely monitored by a doctor to regulate dosage. It may also cause adverse reactions in some people, which should be reported to their doctor immediately.

Phentermine is a stimulant that increases fat metabolism and decreases appetite. As with many stimulants, phentermine can cause rapid heartbeat and an increase in blood pressure. As a result, some individuals can feel dizzy or light-headed while taking phentermine. Nervousness and blurred vision are also common with this type of stimulant, as are stomach upset, diarrhea or constipation. Dry mouth or an unpleasant taste in the mouth has been reported as well.

You may experience some moderate side effects while taking phentermine, such as nausea or vomiting. You may also feel heart palpitations, or experience sleep disturbances such as insomnia. The stimulant effect of phentermine can cause you to feel restless or hyperactive, and can give you a headache. Some individuals even experience impotence while taking this medicine.

Some individuals may experience more severe side effects to phentermine than others. Trouble breathing can occur, along with confusion, severe headaches or even hallucinations. An unexplained fever or sore throat can occur, and skin irritations, such as a skin rash or hives, can develop. Severe chest pain can accompany an irregular, pounding heartbeat. Seizures or uncontrollable movements of the body can also occur. According to the University of Maryland Medical Center, these side effects can indicate a severe allergic reaction to phentermine, so call your doctor immediately if you experience any of these symptoms.

Phentermine may cause negative reactions when taken along with other medications. Side effects can increase if you take phentermine while taking blood pressure medication or insulin for diabetes. You may also experience side effects if you take it with certain antidepressants. According to Drugs.com, there are 36 major drug interactions, 192 moderate drug interactions, and two minor drug interactions that can occur while taking phentermine. Tell your doctor about all medications you are currently taking before you start phentermine.