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Alli & Phentermine

author image Sheri Kay
Sheri Kay has a master's degree in human nutrition. She's the co-author of two books and has been a nutrition and fitness writer since 2004.
Alli & Phentermine
Your doctor can prescribe diet drugs to help you lose weight. Photo Credit Tanya Constantine/Brand X Pictures/Getty Images

Diet drugs are designed to help you lose weight by reducing appetite, speeding your metabolism, blocking nutrient absorption or keep you feeling full longer. Alli and Phentermine are two diet drugs that may be used along with reduced calorie diets or exercise programs, but don’t combine diet drugs without speaking to your doctor.


Alli is the over-the-counter version of Orlistat, a diet drug that’s also available in prescription strength as Xenical. According to Drugs.com, it’s used to treat obesity in people with diabetes, high blood pressure, and high cholesterol and triglycerides. It decreases absorption of some of fat you eat by blocking the enzyme that breaks down fat, called lipase, in your intestinal tract. Reducing the amount of fat you absorb also decreases your caloric intake. A study in the March 2008 issue of "Obesity" reports that at or after 60 days of orlistat use, the average weight loss is 5 percent of starting weight.


Children under 18 years of age should not take Alli. Tell your doctor if you have had thyroid problems, gallstones, pancreatitis, liver disease, or if you take any other medications. Avoid high-fat meals, which can increase the side effects such as oily stools, gas, loose stools, stomach pain, nausea, increased bowel movements and weakness. You may need to take vitamin supplements. Talk to your doctor before taking any over-the-counter or prescription diet drugs.


Phentermine is similar to amphetamine and acts by releasing a chemical called noradrenaline in the brain. The increased noradrenaline level decreases your appetite. Drugs.com says taking Phentermine with other diet drugs fenfluramine or dexfenfluramine can cause pulmonary hypertension. Phentermine may be habit forming and can impair your thinking and reaction times. Consult with your doctor before using any diet drugs and don’t use drugs prescribed for other people.


You should not take Phentermine with other diet drugs or MAO inhibitors, and it may be affected by blood pressure medications, insulin, diabetic medication, guanethidine or some antidepressants. Alcohol may increase the side effects, which include shortness of breath, chest pain, swelling in your feet, racing heartbeat, confusion, mood swings and high blood pressure. Take the correct dosage of phentermine; don’t change the dosage without speaking with your doctor.

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