Phentermine is similar to an amphetamine. It stimulates the central nervous system (nerves and brain), which increases your heart rate and blood pressure and decreases your appetite. Phentermine is used together with diet and exercise to treat obesity, especially in people with risk factors such as high blood pressure, high cholesterol, or diabetes. Phentermine may also be used for purposes not listed in this medication guide.
Phentermine is a prescription weight loss medication. It was approved by the FDA in 1959 for short-term use of up to 12 weeks for people older than 16. In the 1990s, phentermine was combined with other weight loss drugs. This drug combination was commonly called fen-phen. After reports of significant heart problems in users, the FDA pulled the other two drugs used in the treatment — fenfluramine and dexfenfluramine — from the market. Phentermine goes by the brand names Adipex-P, Lomaira, and Suprenza, or you can find it in combination medications for weight loss, such as Qsymia. It’s a controlled substance due to its chemical similarities to the stimulant amphetamine — making it available only with a prescription.
Your doctor may prescribe phentermine if you’re obese, meaning that your body mass index (BMI) is greater than or equal to 30. It may also be prescribed if you’re overweight with a BMI greater than or equal to 27 and have at least one weight-related condition, such as high blood pressure, high cholesterol, or type 2 diabetes.
How it works
Phentermine belongs to a class of drugs called “anorectics,” also known as appetite suppressants. Taking phentermine helps suppress your appetite, thereby limiting how many calories you eat. Over time, this can lead to weight loss. While the exact mechanisms behind the appetite-reducing effects of phentermine remain unclear, the drug is thought to act by increasing neurotransmitter levels in your brain. Neurotransmitters are your body’s chemical messengers and include norepinephrine, serotonin, and dopamine. When your levels of these three chemicals increase, your feeling of hunger decreases. However, you may build a tolerance to the appetite-suppressing effects of phentermine within a few weeks. In that case, you should not increase your dose of the drug but stop using it altogether.
Several clinical studies have proven that phentermine can boost fat loss. The expected average weight loss with phentermine use is 5% of your initial body weight. Yet, over 12 weeks, it can be as high as 10%. This equates to a weight loss of 10–20 pounds (4.5–9 kg) for a 200-pound (90.7 kg) person. In a meta-analysis of six studies, people who took the average dose of 27.5 mg of phentermine for 13 weeks lost an average of 13.9 pounds (6.3 kg) compared to 6.2 pounds (2.8 kg) in placebo groups. While phentermine has been shown to be effective for weight loss, it may work better when combined with topiramate.
Topiramate is a drug that has been used on its own to treat seizures but — like phentermine — also has appetite-reducing properties. Topiramate and phentermine is a combination medication sold under the brand name Qsymia. Compared to three other commonly prescribed drugs for weight loss, the combination of phentermine and topiramate was associated with the highest odds of losing at least 5% of the initial body weight.
What’s more, research suggests that the phentermine and topiramate combination is the most effective weight loss medication to date — with people achieving an average weight loss of 21.6 pounds (9.8 kg) after taking the maximum dose for one year. In users, this weight loss has translated to a significant decrease in waist circumference, improved insulin sensitivity, and blood sugar control, as well as favorable effects on triglyceride and cholesterol levels.
The combination of phentermine and topiramate may help reduce binge eating in people with binge eating disorder (BED) and bulimia nervosa. BED is characterized by eating large amounts of food, often quickly and to the point of discomfort. It’s also associated with a feeling of losing control during the binge, as well as feelings of shame or guilt afterward. Bulimia nervosa involves the same binge-eating behavior as with BED, but includes behaviors, such as self-induced vomiting, in an attempt to compensate for the effects of binge eating.
In a small 12-week study in obese or overweight people with BED, the phentermine and topiramate drug combination was associated with significant reductions in weight, BMI, and binge-eating episode frequency. In another 12-week study, people with BED or bulimia nervosa were randomized to receive the drug combination or a placebo. Over 28 days, treatment with the phentermine and topiramate combination decreased the participants’ number of binge-eating days from 16.2 to 4.2. The same results were not observed in the placebo group. By reducing binging episodes, the drug combination may help the estimated 40–80% of people with BED or bulimia nervosa who are overweight or obese lose weight while improving mood and a sense of control with eating.
Along with its needed effects, phentermine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.
Check with your doctor immediately if any of the following side effects occur while taking phentermine:
Rare side effects like: Seeing, hearing, or feeling things that are not there, severe mental changes.
Side effects of unknown of Incidence: Chest pain, decreased ability to exercise, dizziness, fainting, fast, irregular, pounding, or racing heartbeat or pulse, headache, numbness or tingling in the arms or legs, swelling of the feet or lower legs, trembling or shaking of the legs, arms, hands, or feet, trouble breathing, trouble with thinking, speaking, or walking, weakness
Some side effects of phentermine may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
Side effects of unknown of Incidence: Decreased interest in sexual intercourse, difficulty having a bowel movement (stool), dry mouth, false or unusual sense of well-being, hives or welts, itching, or skin rash, inability to have or keep an erection, increased in sexual ability, desire, drive, or performance, increased interest in sexual intercourse, loss in sexual ability, desire, drive, or performance, redness of the skin, unpleasant taste.
Dosage and precautions
Phentermine alone is designed for short-term use only, as there are no long-term studies on its safety. However, the FDA has approved phentermine in combination with topiramate for long-term use, since the dosage of the two ingredients is lower than the maximum doses of the individual drugs. While serious adverse effects are rare, studies report several side effects of the phentermine and topiramate combination.
The most commonly reported side effects include: Dry mouth, Sleeping problems, Dizziness, Heart palpitations, Flushing of the skin, Fatigue, Constipation, Irritability.
You shouldn’t take phentermine if you have heart disease, hyperthyroidism, glaucoma, or if you’re pregnant or nursing.
Phentermine should also not be prescribed in combination with monoamine oxidase inhibitors (MAOIs), a class of medications used to treat depression. Your doctor will determine whether phentermine is appropriate and safe for you.
Take phentermine exactly as prescribed by your doctor. Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose. Doses are normally taken before breakfast, or 1 to 2 hours after breakfast. Follow your doctor’s dosing instructions very carefully. Never use phentermine in larger amounts, or for longer than prescribed. Taking more of this medication will not make it more effective and can cause serious, life-threatening side effects.
This medicine is for short-term use only. The effects of appetite suppression may wear off after a few weeks. Phentermine may be habit-forming. Misuse can cause addiction, overdose, or death. Selling or giving away this medicine is against the law. Call your doctor at once if you think this medicine is not working as well, or if you have not lost at least 4 pounds within 4 weeks.
Do not stop using this medicine suddenly, or you could have unpleasant withdrawal symptoms. Ask your doctor how to safely stop using this medicine. Store at room temperature away from moisture and heat. Keep the bottle tightly closed when not in use.