How long can you take phentermine for weight loss?

Key takeaways

• The FDA has approved phentermine for weight loss for up to 12 weeks. Some healthcare professionals may prescribe it longer for certain off-label uses.
• Studies suggest phentermine is most effective between 3 and 12 months of treatment. Continuing treatment beyond 12 months up to 24 months often leads to notable weight regain.
• Long-term phentermine treatment may pose risks, including high blood pressure, heart disease, irregular heartbeat, seizures, difficulty sleeping, anxiety, and other side effects. However, there isn’t enough evidence on its long-term safety.

Overview

Phentermine is a prescription medication that healthcare professionals prescribe to help you lose weight. It belongs to a class of drugs called sympathomimetic amines, also known as anorectics.

Anorectic medications suppress your appetite. Phentermine likely works by stimulating your nervous system and increasing chemical messengers like norepinephrine in your brain. 

In 1959, the FDA approved phentermine for weight loss, paired with increased exercise and a reduced-calorie diet. 

Past and current phentermine brand names in the United States include: 

• Adipex-P 
• Fastin 
• Ionamin
• Lomaira
• Obe-Nix
• Suprenza
• Zantryl

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Is there a limit to how long you can take phentermine?

The FDA approved phentermine for short term treatment of no more than a few weeks. Some healthcare professionals may prescribe it longer for different reasons. This is called off-label medication use. 

In 2012, the FDA also approved a medication called Qsymia for weight management. It contains both phentermine and another drug called topiramate. This allows for weight loss treatment for more than 12 weeks. 

You should only take phentermine for as long as your healthcare professional has prescribed. 

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What is the risk of long-term phentermine treatment?

Taking phentermine for longer than 12 weeks (FDA has approved it for this duration) may involve some risks. You should be aware of them and discuss them with a healthcare professional.

Dependence

Phentermine is classified as a Schedule IV Controlled Substance. This means it has a chance of misuse and dependence.

Phentermine is chemically related to amphetamine, a stimulant drug that may cause dependence, especially with long-term treatment. 

As per a 2024 research, there is no conclusive evidence that suggests that phentermine treatment can cause addiction. Most healthcare professionals prefer short-term treatments only.

Other risks

The 2022 Clinical Practice Guidelines list the following potential risks of phentermine treatment, which may apply to both short- and long-term treatment:

• high blood pressure
• heart disease
• increased heart rate
• irregular heartbeat
• seizures
• difficulty sleeping 
• constipation
• irritability 
• anxiety 
• headaches
• nausea

The authors note that studies on the long-term safety and efficacy of phentermine and the likelihood of these symptoms are lacking.

Should you take periodic breaks from phentermine when taking it long term?

You should follow the instructions from your prescribing healthcare professional. They will weigh up the benefits and risks of this medication and recommend suitable treatment periods, which may or may not include periodic breaks. 

Besides potential health risks introduced earlier, other reasons why you may need to stop taking phentermine or take breaks from it include:

• Taking other medications: According to a 2023 study, phentermine can have potentially dangerous interactions with certain drugs. If you need to take any of these, you may need to stop phentermine treatment, either short-term or permanently.
• Pregnancy and nursing: Doctors generally recommend avoiding phentermine during pregnancy as weight loss may be harmful to fetuses. A 2019 case report suggests the drug can harm fetuses. So, it may be safest to stop phentermine treatment in these cases. Always consult with your healthcare professional if you are planning to become pregnant.
• Alcohol use: The drug’s prescribing information notes that phentermine could cause adverse reactions if you take it with alcohol. If you wish to consume alcohol, a healthcare professional may recommend stopping phentermine treatment. 

How soon does phentermine work?

Phentermine affects your body quickly. According to the drug label, it only takes around 3 to 4 hours for the medication to reach peak concentrations in your body after taking a pill. This means you might experience the appetite-suppressing effects within the first day of treatment, though this may vary from person to person.

As for noticeable weight loss, results may also differ between individuals. For example, how well you stick to lifestyle changes like regular physical activity will play a significant role in how soon you lose weight and body fat. 

A 2019 study of electronic health record data looked at how effective phentermine was at different lengths of treatment. It found that many people did not experience lasting or significant weight loss when taking phentermine for 3 months or less. Meanwhile, when doctors prescribed it off-label for up to 12 months, it led to greater weight loss. 

However, continuing phentermine treatment for longer than 12 months and up to 24 months often led to notable weight regain. 

This suggests that for many people, phentermine is most effective within 3 to 12 months of treatment. 

It’s best to work with a healthcare professional to monitor your progress while taking phentermine. They can advise if it’s safe and effective for you or recommend other options like alternative weight loss medications. 

Other weight loss medications

While phentermine is a popular choice, there are many other weight loss medications available that can support your eating habits and exercise efforts to lose weight. 

A few examples introduced in a 2023 review are:

• Orlistat (Xenical) blocks the absorption of dietary fat. Orlistat is also available as an over-the-counter product under the brand name Alli, although in a lower dose.
• Liraglutide (Saxenda) slows how fast your stomach empties after eating and suppresses your appetite.
• Bupropion and naltrexone (Contrave) is a combination of drugs that affects the reward centers in your brain and helps reduce cravings.
• Semaglutide (Wegovy, Ozempic) regulates blood sugar levels and promotes weight loss by increasing feelings of fullness.
• Tirzepatide (Mounjaro) works by decreasing your appetite and slowing down the movement of food through your digestive system.

A healthcare professional can help identify the best approach for your health and goals.

Online care for weight loss medications

Weight loss medications are safe and effective, but many people may be reluctant to seek them out due to feelings of shame or discomfort.

Online care services, including Optum Perks Online Care, may make getting treatment easier. With these services, you can submit an online form answering a few questions about your symptoms, and a US-licensed healthcare professional will respond soon after to prescribe a treatment plan.

Summary

Phentermine can help you lose weight, but there is little evidence of its safety for long-term treatment. It may be most effective between 3 and 12 months of treatment. 

Understanding the risks and benefits of phentermine and exploring alternative medications under the guidance of a healthcare professional ensures a safer and more effective weight loss journey. 

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