Phentermine: Side effects, history & dosage – The ultimate guide

What is phentermine?
How does phentermine work?
When will phentermine be prescribed?
The history of phentermine
Does phentermine have side effects
When not to take phentermine
Phentermine abuse
Phentermine dosage
How much does phentermine cost?
Other weight loss pills
The natural, prescription-free alternative

What is phentermine?

Phentermine is a stimulant similar to amphetamine and is often prescribed to help treat obesity. Because it works as an effective appetite suppressant, phentermine has become the most popular weight loss drug in the US, marketed under brand names like Adipex-P and Suprenza. It’s currently only available with a prescription, but there are a lot of different over-the-counter phentermine combinations and alternatives available today. To understand it’s popularity, we’ve taken an in-depth look at America’s favorite diet pill.

How does phentermine work?

Phentermine works as an anorectic (or anorexic), which is an agent that suppresses your appetite. The drug does this by stimulating the neurons in your brain to release a type of neurotransmitter called catecholamines, which include dopamine and norepinephrine. High levels of these catecholamines work to stop hunger signals, which helps to diminish your appetite. So, when your body’s telling you it’s hungry, your brain doesn’t get the message.

One of the reasons for this chemical reaction is that norepinephrine and dopamine are involved in your brain’s stress responses. So, when these neurotransmitters are released, they signal the fight-or-flight response in your body. This reaction in your body can cause your heart rate and blood pressure to increase, and also lead to a reduction in your appetite.

Many clinical trials that study the before and after effects of phentermine have shown that, when compared to placebos and other weight loss drugs, phentermine helps achieve a greater weight loss. A review in The JAMA Network analysed 28 of these clinical trials, which studied a total of nearly 30,000 overweight adults. The authors found that 75% of participants taking phentermine achieved at least a 5% weight loss, compared to the 23% taking a placebo. The next highest-performing drug was liraglutide, which found 63% of participants lost 5% of their body weight.

When will phentermine be prescribed?

Because of its effect as an anorectic, phentermine is prescribed to help people kickstart their weight loss. However, it’s generally only prescribed to patients struggling with obesity, which means having a body mass index (BMI) of 30 or over. If a patient has other risk factors, such as controlled hypertension, hyperlipidemia or diabetes, they may be prescribed phentermine with a BMI of 27 or higher.

To get the greatest effect from phentermine, it’s prescribed as an adjunct to a healthier lifestyle. It’s recommended that patients taking phentermine also drop their calorie intake and adopt a more active routine. Those who rely solely on the drug are unlikely to lose as much weight as those who also adopt a healthier diet.

Doctors are often hesitant to prescribe phentermine, and will first encourage patients to adopt a more active routine. This has led to a surge in the popularity of natural, prescription-free phentermine alternatives, which can be taken alongside a healthy lifestyle to boost weight loss. If your BMI doesn’t fall into the obese category, you should consider an over-the-counter alternative to phentermine.

The history of phentermine

Phentermine was introduced in the 1950s as a weight loss drug, and was originally approved in the US in 1959 by the Food and Drug Administration (FDA). It gradually became available in a hydrochloride salt and resin form referred to as phentermine hydrochloride. It was marketed with either fenfluramine or dexfenfluramine, both of which also act as appetite suppressants. Together, these medicines were advertised as Fen-Phen, a combination drug to fight obesity, burn fat and suppress appetites.

In 1997, fenfluramine and dexfenfluramine were taken off the market at the request of the FDA, after it was found that there were 24 cases of heart valve disease in Fen-Phen users.The decision to remove the drugs was made after studies using echocardiograms – a scan which tests the functioning of heart valves – showed that 30% of people evaluated had abnormal test results. This was a much higher result than they’d originally expected.

The FDA did not ask for phentermine to be withdrawn from the market, and it remains widely available in the US. It’s classed as a controlled substance by many countries, including America, due to its similarities to the stimulant amphetamine. Because of this, phentermine as a single drug is currently only available with a prescription. However, there are many natural alternatives to phentermine that are available without a prescription.

Does phentermine have side effects?

Like most medicines, phentermine can have some unwanted side effects. Not all of these are serious, and some may go away as you get used to the medicine, but you should always check with your doctor if your symptoms persist or get worse. Many people opt for natural weight loss pills instead of prescription drugs to avoid phentermine side effects.

Some of the more common side effects of phentermine include vomiting, constipation, diarrhoea, insomnia, headaches, an increased or decreased sex drive, a bad taste in the mouth and feeling hyperactive. Speak to your doctor if any of these symptoms are causing you severe discomfort, or are affecting your day-to-day activities.

It’s rare to have serious side effects when you’re taking phentermine, but if you do have a severe reaction it’s important to seek medical attention immediately. The more serious side effects include severe headaches, dizziness, chest pain, seizures, difficulty breathing and an irregular or pounding heartbeat.

It’s also important to be aware of the impact that taking phentermine can have on your mental health. Because of how phentermine works, it’s often referred to as a psychostimulant, producing a temporary increase in mental or physical functions. Other psychostimulants include nicotine, methamphetamine, Ecstasy and cocaine.

Like other psychostimulants, phentermine can cause both minor and severe mental health side effects. These include agitation, anger, anxiety and depression. Depression is also a symptom of phentermine withdrawal, so it’s important to monitor your mood if you start or stop taking the drug.

If you notice any dramatic physical or mental changes when you’re taking phentermine, contact a medical professional right away. If you have any minor side effects that are persistent or causing you discomfort, contact your doctor. They may be able to prescribe an alternative medicine or give you something to ease your discomfort.

When not to take phentermine

Phentermine works great as a weight loss drug, but it isn’t suitable for everyone. You shouldn’t take it if you’re pregnant or are breastfeeding, or if you have a history of very high blood pressure, heart disease, glaucoma, an overactive thyroid, or a history of pulmonary hypertension.

It also shouldn’t be taken if you’ve had a monoamine oxidase inhibitor (MAO) in the last two weeks. These are used to treat anxiety and depression, and are generally only prescribed when other antidepressants have proved ineffective. Combining phentermine with MAO inhibitors, such as linezolid, rasagiline and isocarboxazid, can cause a dangerous drug interaction.

Phentermine abuse

If you have a history of drug abuse, you shouldn’t take phentermine. Although any prescription drug can be addictive, stimulants like phentermine can be particularly addictive because of the effects they have on your mind and body. Users can get addicted to the feeling of increased focus and energy, but also to phentermine’s effects as an anorectic. Because it’s a simple way to lose weight, people trying to reach their ideal body weight quickly often take more than the recommended dosage, and find themselves becoming dependent on the drug.

The best way to avoid becoming dependent on phentermine is to take it as directed, and to call your doctor if you notice any side effects or you feel yourself becoming addicted. If you do have a history of drug abuse, consider taking an over-the-counter option to help with your weight loss journey.

Phentermine dosage

The recommended adult dose for phentermine is 8 mg orally three times a day, 30 minutes before meals. Alternatively, a 15 to 37.5 mg dosage can be taken once a day, either before breakfast or a couple of hours after breakfast.

Regardless of which dosage you have been prescribed, you should try and avoid taking phentermine late at night. This is because one of the possible side effects of the drug is insomnia, and taking your dose as early in the day as possible reduces the risk of sleep difficulties.

How much does phentermine cost?

The price of phentermine can vary, depending on which pharmacy you visit and what dosage you’ve been prescribed. For oral capsules, the price starts at $17.09 for a pack of seven 15 mg capsules, and a pack of 37.5 mg capsules is available from $20.53. Phentermine oral tablets are slightly cheaper than the capsules, and a pack of seven tablets starts from $10.40.

Other weight loss pills

Many weight loss drugs, including phentermine, work by suppressing your appetite. Though phentermine is the most popular and most frequently-prescribed drug in America, there are lots of other prescription medicines that also work as an anorectic. We’ve had a look at some of the other popular weight loss drugs, and compared their similarities and differences to phentermine.

Phendimetrazine: Brand names for phendimetrazine include Adipost, Bontril PDM and Plegine. It’s a sympathomimetic amine that also works as an anorectic, and it’s often prescribed to people for weight loss. However, it operates differently to phentermine. Whilst phentermine affects your hormone levels, phendimetrazine works by stimulating the central nervous system, which increases your heart rate and reduces your appetite. Similarly to phentermine, you shouldn’t take it if you have a history of drug or alcohol abuse.

Methamphetamine: Sold under the brand name Desoxyn. Methamphetamine is a powerful central nervous system stimulant and is often used to treat attention deficit hyperactivity disorder (ADHD). Similarly to phentermine, it is sometimes prescribed to treat obesity. However, because of how powerful and addictive methamphetamine is, it will only be recommended as a last resort after other methods and medicines have failed. It has an extremely high abuse potential, and shouldn’t be used by anyone with a history of drug or alcohol abuse.

Benzphetamine: Marketed under the names Rigimex, Didrex and Recede. Both benzphetamine and phentermine are amphetamines that have an anorectic effect, and they have a similar impact on your body. They also have similar warnings attached, and neither should be taken if you’re pregnant or currently breastfeeding. However, there are some differences in potency and recommended dosage. Whilst a 50 mg dose of Benzphetamine can be taken up to three times a day, the recommendation for phentermine is up to 37.5 mg once a day.

Diethylpropion: Sold under brand names like Tenuate, Tenuate Dospan and Tepanil. Diethylpropion is a stimulant and, like phentermine, it works as an effective appetite suppressant. However, clinical studies have shown that phentermine is more effective as a weight loss aid than diethylpropion. A study in the National Library of Medicine, which compared the effectiveness of phentermine and diethylpropion for treating obesity, found that there was a significantly greater weight loss in participants taking phentermine.

Topiramate: Topiramate is an anticonvulsant (also known as antiepileptic or antiseizure drugs). It’s often combined with other medicines, including phentermine. The combination of phentermine and topiramate is marketed under the brand name Qsymia. Similarly to phentermine, it’s used alongside exercise and a healthy diet to treat obesity. Many patients may prefer phentermine as a single drug, as generic phentermine drugs are often cheaper than Qsymia. However, unlike phentermine, the combination drug is approved for long-term use. This is because the strength of phentermine in Qsymia is much lower than in generic phentermine drugs, starting at a 3.75 mg -23 mg dose (phentermine to topiramate).

Orlistat: Sold under the brand names Alli and Xenical. Like phentermine, it’s sometimes prescribed alongside a low-calorie diet to significantly overweight patients. However, orlistat isn’t an anorectic and doesn’t work to suppress your appetite. Instead, it works to block some of the fat you eat and keeps it from being absorbed by your body. Both orlistat and phentermine are proven to be effective weight loss aids, but phentermine is only recommended for short-term use. If it proves effective for treating obesity, orlistat can be prescribed indefinitely. However, this can incur huge costs, as orlistat is more expensive than phentermine.

The natural, prescription-free alternative

Now that you know a little more about phentermine, you can make an informed decision on which weight loss method you’d like to choose. However, many doctors are unwilling to prescribe phentermine, and will usually only recommend it if you’re struggling with obesity. If your BMI doesn’t fall into the obese category, or if you’d like to avoid the side effects of prescription drugs, you should consider a natural alternative.

That’s where PhenQ comes in.

PhenQ is a natural, prescription-free alternative to phentermine, and combines the effects of both an appetite suppressant and a fat burner into one handy pill. Because it’s made of 100% natural ingredients, it’s easy to buy and completely safe to use, making it a great option for people who can’t get prescription drugs.

Similarly to phentermine, PhenQ is recommended as an adjunct to a healthy lifestyle. It works alongside your active routine to suppress your appetite, help your body burn more fat and boost your energy levels. This leads to a greater overall weight loss, and leaves you with more energy to workout regularly. PhenQ is also a lot cheaper than phentermine, so it’s worth considering an over-the-counter option before you look at prescription-strength drugs.




  1. Phentermine: Drug Uses, Dosage & Side Effects, (2018). Accessed 8 Feb 2018.
  2. Anorectic: Definition of anorectic, TheFreeDictionary’s Medical Dictionary (2018). Accessed 8 Feb 2018
  3. Phentermine: C10H15N, PubChem Compound Database (2018). Accessed 8 Feb 2018.
  4. Association of Pharmacological Treatments for Obesity With Weight Loss and Adverse Events, R. Khera et al, The JAMA Network  (2016). Accessed 8 Feb 2018.
  5. Phentermine: FDA Prescribing Information, Side Effects & Uses, (2018). Accessed 8 Feb 2018.
  6. Long-term Drug Treatment for Obesity: A Systematic and Clinical Review, S. Yanovski, M.D. and J. Yanovski, M.D., PhD, PubMed Central (2014). Accessed 8 Feb 2018.
  7. Adipex-p (phentermine hydrochloride) capsules label, U.S. Food & Drug Administration (2012). Accessed 8 Feb 2018.
  8. Handbook of Obesity – Volume 2: Clinical Applications, G. Bray and C. Bouchard, Taylor & Francis Group (2014). Accessed 8 Feb 2018.
  9. Convention on Psychotopic Substances, United Nations (1971). Accessed 8 Feb 2018.
  10. Phentermine Side Effects & Adverse Reactions, Mental Health Daily (2018). Accessed 8 Feb 2018.
  11. Psychostimulant: Definition, (2018). Accessed 8 Feb 2018.
  12. Effects of Phentermine Diet Pill Withdrawal, T. Hanes, (2017). Accessed 8 Feb 2018.
  13. Phentermine and Addiction, Drug Addiction Treatment (2014). Accessed 8 Feb 2018.
  14. Phentermine Dosage Guide with Precautions, (2018). Accessed 8 Feb 2018.
  15. Phentermine Prices, Coupons and Patient Assistance Programs, (2018). Accessed 8 Feb 2018.
  16. Phendimetrazine: Brand Name List, (2018). Accessed 8 Feb 2018.
  17. Methamphetamine: Brand Name List, (2018). Accessed 8 Feb 2018.
  18. Methamphetamine Hydrochloride: Drug SummaryPrescribers’ Digital Reference (2018). Accessed 8 Feb 2018.
  19. Benzphetamine: Brand Name List, (2018). Accessed 8 Feb 2018.
  20. Benzphetamine Dosage Guide with Precautions, (2018). Accessed 8 Feb 2018.
  21. Diethylpropion: Brand Name List, (2018). Accessed 8 Feb 2018.
  22. A comparative study of phentermine and diethylpropion in the treatment of obese patients in general practice, JC. Vallé-Jones, NH. Brodie, H. O’Hara, J. O’Hara and RL. McGhie, PubMed Central (1983). Accessed 8 Feb 2018.
  23. Phentermine/Topiramate Dosage Guide with Precautions, (2018). Accessed 8 Feb 2018.
  24. Orlistat: Uses, Dosage & Side Effects, (2018). Accessed 8 Feb 2018.
  25. Orlistat, General Practice Notebook (2018). Accessed 8 Feb 2018.


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