Is Phentermine a Good Option For Weight Loss?


Written by Elite HRT on November 09, 2021

Medically reviewed by

Camille Freking, Regulatory Affairs Specialist, MEDICAL ADVISOR

Phentermine is a prescription medication that suppresses appetite to assist with weight loss. For people who have a weight-related medical condition or are obese, taking phentermine can be life-changing.  

Phentermine is an inexpensive drug that is meant to be taken on a short-term basis (approximately 3 months). Read on to learn more about phentermine, how it works, and potential side effects.  

What is Phentermine?

Phentermine is a prescription weight-loss pill that was first introduced in 1959 to help combat obesity. As the oldest prescription diet pill, it continues to be used to help people lose weight quickly and safely. It is in a class of medications called anorectics and works by decreasing your appetite. 

Phentermine is a controlled substance, as it has chemical similarities to amphetamine (a stimulant), and it is only accessible with a prescription. It is available under the brand names Adipex-P, Lomaira, and Suprenza. 

How Does Phentermine Work for Weight Loss?

Phentermine hydrochloride is an anorectic medication, which means it helps reduce appetite. Phentermine helps manage hunger through effects in the central nervous system. It works by increasing the activity of certain neurotransmitters in the hypothalamus, which helps control hunger. 

People who take phentermine tend to feel hungry less often, resulting in them eating fewer calories. This leads to a caloric deficit — meaning they are burning more calories than they are eating. When combined with a healthy diet and exercise, this can lead to significant weight loss with only short-term use. 

If you’re trying to lose weight but lifestyle changes haven’t helped, phentermine may be a good option for you. Particularly if you are obese or have weight-related health conditions, your doctor may prescribe this medication to give your brain and body the boost it needs to help you shed the extra pounds and improve your overall health. 

Is Phentermine Safe?

When taken as prescribed on a short-term basis and in the correct dosage, phentermine is a safe and effective medication to help with weight loss. Phentermine may have a maligned reputation due to its risk and history of drug abuse. 

In the 1990s, phentermine’s popularity skyrocketed. Millions of Americans were prescribed a weight-loss therapeutic medication called fen-phen. This medication was a combination of two diet drugs, fenfluramine and phentermine. The popularity continued until the U.S. Federal Drug Administration (FDA) pulled the combo-medication from the market, as it was linked to heart and lung problems in some people.  

However, phentermine alone (without being paired with fenfluramine) remains available as a prescription medication. When taken on a short-term basis (3 months), phentermine is considered to be a safe and effective option to aid in weight loss. Some providers may prescribe phentermine for a longer period of time. 

Chemically, phentermine is similar to amphetamines, as it has stimulant properties. The FDA has classified phentermine as a Schedule IV drug due to concerns of potential addiction or abuse. 

Phentermine should only be taken if you are obese with a body mass index over 30, or if you have a serious medical condition related to weight, such as high blood pressure or diabetes. 

The real danger of phentermine lies not in the medication itself, but in how it may be misused and mishandled. Be sure to take it exactly as prescribed, and ensure you are getting phentermine from a reliable source (do not buy it online without a prescription). 

What Are Phentermine’s Side Effects?

While generally safe, there are some common side effects you may experience when taking phentermine. 

Common side effects of phentermine include:

  • Constipation 
  • Dry mouth
  • Nervousness or restlessness
  • Rapid heart rate 
  • Tingling in hands or feet
  • Unpleasant taste in your mouth
  • Vomiting
  • Nausea 
  • Diarrhea

Some people may experience more serious side effects when taking phentermine. If you experience any of these symptoms, which may indicate overdose, stop taking the medication and call your doctor: 

  • Chest pain 
  • Severe headache
  • Confusion
  • Difficulty engaging in physical activities you usually do
  • Dizziness 
  • Edema (swelling in your legs and feet) 
  • Insomnia 
  • Hallucinations
  • High blood pressure (pulmonary hypertension)
  • Irregular heartbeat
  • Shortness of breath
  • Tremors or seizures

Do not take phentermine if you have an allergy to any ingredients in the medication, heart disease (including valvular disease or heart failure), history of stroke or heart rhythm problems, high blood pressure, glaucoma, an overactive thyroid, or if you are pregnant, trying to become pregnant, or are breastfeeding. You should also avoid taking phentermine during or within 14 days after taking a monoamine oxidase inhibitor (MAOI) medication. 

Who is Phentermine Typically Prescribed To?

Phentermine is typically prescribed to obese individuals who have a body mass index (BMI) of 30 or higher. It is also prescribed to individuals who are overweight and have weight-related health conditions, such as heart disease and diabetes, and thus may benefit from appetite suppressants. 

If you’re slightly overweight and just need a little help fitting into your skinny jeans, phentermine is not a healthy option. Instead, try reducing your caloric intake, eating whole, fresh foods, and exercising for 30 minutes a day, at least 5 days a week. 

Phentermine is considered to be a low-cost solution, making it accessible to individuals who are low-income or may not otherwise be able to afford expensive prescription medications to help them shed pounds. 

The decision to take phentermine should be made with your healthcare provider. Being aware of the potential side effects can help you determine if this is the right medication for you. 

How Much Weight Will I Lose With Phentermine?

Highly effective, phentermine may help you lose 5 to 10% of your body weight in 3 to 6 months or more when used with a low-calorie diet and a regular exercise program. In a person who weighs 200 pounds, this is a weight loss of 10-20 pounds. 

Studies show that people who take an average dose of phentermine (27.5 mg/day) for 13 weeks lost an average of 13.9 pounds. That said, how much weight a person loses while taking phentermine is a highly individual experience. It is important to combine the medication with a healthy diet and regular physical activity. 

While phentermine is meant to be a short-term solution for weight loss, it can be combined with another medication for long-term use. Phentermine is often combined with another medication known as topiramate. The combination medication is known as Qsymia and is approved for long-term use to assist with weight loss. One study found that 75% of people taking Qsymia for a year lost 5% or more of their body weight. 

Experts suggest that phentermine and topiramate combined is the most effective weight loss medication available. Many people lose over 20 pounds after taking the maximum dose of the medication for a year. This would not only help you feel better in your body, but may improve insulin sensitivity, control blood sugar levels, and may help reduce cholesterol levels. 

Healthy Ways To Promote Weight Loss

Phentermine can be an effective weight-loss aid, but there are other tried and true methods to help you shed unwanted pounds. Without making changes to the way you eat and live, you may gain back the weight you lost while taking phentermine. 

Healthy lifestyle behaviors are the best way to lose weight and keep it off long term. These include: 

  • Calorie reduction. If you have weight to lose, aim to eat 500 fewer calories each day. 
  • Eating a healthy, balanced diet. Focus on eating nutrient-dense foods rich in protein, vitamins and minerals. 
  • Increase physical activity. Get at least 150 minutes of exercise each week. This may include walking, cycling, weight lifting, and high-intensity interval training (HIIT). 
  • Drink plenty of water. Try to drink at least eight 8-ounce glasses of water each day. This can not only decrease your appetite but will give your body the hydration it needs to function properly. 

Making lifestyle changes can be challenging. It requires an investment of time and commitment to stick with these changes to see the changes you want to see in your body. If you implement healthy lifestyle changes, long-term weight loss and improving your overall health and wellness are within reach. 


When combined with a reduced-calorie, nutrient-dense diet and exercise, phentermine is a safe medication to consider to assist with weight loss. It’s important to eat a healthy diet and increase your physical diet when on this medication. Without healthy lifestyle behaviors, you’re likely to gain back all the weight you lost after you stop taking phentermine. 

Phentermine is only meant to be used on a short-term basis to aid in weight loss. Your diet and physical activity levels are most important for long-term success. 

With appropriate medical supervision, phentermine can be a highly effective part of your weight loss program. Talk with your doctor today to ask if phentermine is an option for you. 


Topiramate And Phentermine | NCBI

Phentermine For Weight Loss Seems Safe, Effective for Longer-Term | Kaiser Permanente

The Problem with Fen-Phen | JAMA Network

Phentermine | Medline Plus

Pharmacotherapy for obesity: a quantitative analysis of four decades of published randomized clinical trials | National Library of Medicine 

Association of Pharmacological Treatments for Obesity With Weight Loss and Adverse Events: A Systematic Review and Meta-analysis | National Library of Medicine