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Is Phentermine a Safe Weight Loss Treatment?

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Eating a healthy, well-balanced diet and getting regular exercise are the safest and most effective ways to lose weight. However, a weight loss medication like phentermine can be a useful adjunct in people for whom diet and exercise alone have not been successful. While phentermine is one of the most well-known weight loss medications in the world, it does carry some risks. In this article, we’ll talk about how phentermine helps you get weight loss results, how long you can take this medicine safely, as well as the benefits, risks, and possible side effects of this medicine.

What is phentermine?

Phentermine is an FDA-approved prescription medication for appetite suppression and weight loss. It is available under the brand names Lomaira, Adipex-P, and Suprenza. 

Phentermine is also found, along with topiramate, in the combination weight loss medication Qsymia

Phentermine for weight loss is a controlled substance because it is chemically similar to amphetamine, which is a stimulant. 

Who can take phentermine?

Phentermine is an FDA-approved drug for short-term weight management in people over 16 years of age. 

Other weight loss medications such as orlistat (Xenical) and liraglutide (Saxenda) are approved by the Food and Drug Administration (FDA) for use in adults and children over the age of 12 years.

Doctors prescribe phentermine to people who are unable to lose weight with dietary and lifestyle changes alone. The goal is to reduce the risk of health problems related to being overweight and obese. 

Your healthcare provider will evaluate your BMI to determine if phentermine is an appropriate choice for you. Per guidelines recommendation, phentermine can be prescribed to adults with a BMI of 30 or higher or a BMI of 27 or higher with a weight-related condition such as high blood pressure or diabetes.

It is worth noting that weight management with medications is not right for everyone with a BMI over 30. Some people are able to achieve their goal weight with a strict reduced-calorie diet and lifestyle program that includes consistent, high-intensity to regular physical activity. 

How does it work?

Phentermine is an anorectic (appetite suppressant). It helps you lose weight by suppressing your appetite and thus limiting the number of calories you eat. Over time, your low-calorie diet leads to a reduction in body weight.

Scientists do not fully understand how phentermine suppresses the appetite. Though, it is believed that phentermine works by increasing the levels of certain brain chemicals (neurotransmitters). When levels of neurotransmitters like dopamine, serotonin, and norepinephrine are high, you feel less hungry.

It is worth noting that you can develop a tolerance to phentermine. Meaning, within a few weeks, you may need to increase the dose to get the same appetite-suppressing effects. If this occurs, you should consult a healthcare provider for professional medical advice. Do not increase your dose without talking to a doctor first.

How long can you safely take phentermine?

Phentermine should be used for the short-term only (less than 12 weeks at a time). There are no studies on its long-term safety. 

Notably, the FDA has approved the combination of phentermine and topiramate (brand name Qsymia) for long-term use. This combination contains both drugs at lower doses. 

The combination of phentermine and topiramate is also used to treat people with binge eating disorder (BED) and bulimia nervosa.

How much weight can you lose on phentermine?

On average, people lose 5% of their initial body weight with phentermine, but it can be as high as 10% over a 12-week period. This equals a weight loss of 10-20 pounds for a 200-pound individual. 

What are phentermine side effects?

Serious adverse effects from phentermine are rare. The most commonly reported side effects include:

  • Dry mouth
  • Dizziness
  • Sleeping trouble
  • Heart palpitations
  • Skin flushing
  • Tiredness
  • Irritability
  • Constipation

Who should not take phentermine?

Phentermine is not the right choice for everyone. If you have heart disease, high blood pressure, glaucoma (increased eye pressure), or hyperthyroidism (overactive thyroid), you should not take this medication. Pregnant women and nursing mothers should not take phentermine. 

Phentermine can have drug interactions with certain other medications. You should not take phentermine if you are taking monoamine oxidase inhibitors (MAOIs) (this is an older class of prescription medications used to treat depression).

Your doctor will obtain a complete medical history and determine whether phentermine is an appropriate and safe weight management option for you.

What are the disadvantages of phentermine?

The diet pill phentermine has a bad reputation mainly for two reasons:

Phentermine is chemically similar to amphetamines

The U.S. Food and Drug Administration (FDA) has classified phentermine as a Schedule IV controlled substance. This is because the chemical structure of phentermine is similar to amphetamines, which are stimulant drugs with a potential for abuse and addiction. However, phentermine is not an amphetamine. Studies have shown that abuse or addiction does not develop in patients treated with phentermine for weight loss. Treatment with this medication has not been found to induce phentermine drug cravings, a hallmark of addiction. Also, there are no withdrawal symptoms if phentermine is stopped abruptly, even after long-term use. 

Phentermine was in phen-fen

Phentermine has been combined with other weight loss drugs in the past. In the 1990s, one such combination gained massive popularity as a weight-loss drug called phen-fen. It contained phentermine and fenfluramine or dexfenfluramine, hence the nickname phen-fen. In 1997, the FDA pulled phen-fen from the market when it was discovered that the fenfluramine and dexfenfluramine components were causing serious heart problems. However, phentermine itself was deemed safe.

Phentermine safety: The bottom line

Phentermine is not a quick fix or a replacement for diet and exercise. You should only consider phentermine or other weight loss medications if you have a body mass index (BMI) over 30 or a BMI over 27 and a serious weight-related medical condition, such as diabetes or high blood pressure.

Phentermine is safe when used under medical supervision and as part of an overall plan for weight management that includes lifestyle changes. Obtaining phentermine from non-medical sources and looking at it as a fast and easy weight loss solution can be very risky, not to mention illegal. Keep in mind that without behavioral and lifestyle changes to stay at a healthy weight, you will likely gain back all the weight you lost after you stop taking phentermine. 

If you keep these caveats in mind, phentermine can be a highly effective adjunct to your weight loss program.



  1. https://www.mayoclinic.org/healthy-lifestyle/weight-loss/expert-answers/phentermine/faq-20057940
  2. https://www.niddk.nih.gov/health-information/weight-management/prescription-medications-treat-overweight-obesity
  3. https://pubmed.ncbi.nlm.nih.gov/23736363/