side effects of phentermine

Your complete guide to phentermine side effects, history & dosage

11 min read


10 May 2018

Phentermine is a psychostimulant drug which has been successfully used as an appetite suppressant for short term use since 1959. Although it’s still widely available for sale in most countries, it’s safety has been questioned over the years.

If you’re thinking of taking phentermine for weight loss, make sure you know the full facts first. We’ve summed up everything you need to know right here.

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 chemical structurePhentermine 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. This happens in 3 stages;

  1. Phentermine stimulates the neurons in your brain to release a type of neurotransmitter called catecholamines, which include dopamine and norepinephrine.
  2. High levels of these catecholamines work to stop hunger signals, which helps to diminish your appetite.
  3. As a result, when your body’s telling you it’s hungry, your brain doesn’t get the message.

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% not taking it.

When will phentermine pills be prescribed?

Because of its effect as an anorectic, phentermine pills are prescribed to help people kickstart their weight loss. However, it’s generally only prescribed to patients with 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 lower BMI of 27-19.

Body mass index

To get the greatest effect from phentermine, it’s recommended that patients also drop their calorie intake and adopt a more active lifestyle. 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 diet pills, due to the potential side effects a patient may experience. They will carefully weigh up the potential risks against the potential benefits, and will be likely to encourage you to attempt to lose the weight by natural means first of all if at all possible. This reluctance 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. PhenQ is just one example.

PhenQ - prescription free phentermine alternative

The history of phentermine

Phentermine was first approved for use as a weight loss drug 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 blended with both fenfluramine and dexfenfluramine, both of which also act as appetite suppressants. This drug was marketed as Fen-Phen, a combination drug to fight obesity, burn fat and suppress the appetite.

However, 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 FDA didn’t ask for phentermine as a substance in its own right to be withdrawn from the market however, and it remains widely available in most countries. Having said this, it’s still 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.

Does phentermine have side effects?

Like most medicines, phentermine can have some unwanted side effects. Not all of these are serious, and some may decrease as you get used to the medicine, but you should always check with your doctor if your symptoms persist or worsen.

Some of the more common side effects of phentermine include:

  • vomiting
  • constipation
  • diarrhoea
  • insomnia
  • headaches
  • Increased or decreased sex drive
  • a bad taste in the mouth
  • hyperactivity

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
  • 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
  • cocaine

Like other psychostimulants, phentermine can cause both minor and severe mental health side effects.

These include:

  • agitation,
  • anger
  • anxiety
  • depression

Depression is also a common side effect of phentermine withdrawal, so it’s important to monitor your mood if you stop taking the drug.

If you notice any dramatic physical or mental changes while 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.

Lose weight without side effects

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 take it if any of the following apply to you:

  • You’re pregnant or breastfeeding
  • You suffer with very high blood pressure
  • You have heart disease
  • You have glaucoma
  • You have an overactive thyroid
  • You have 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 proven 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.

Phentermine dosage

The recommended adult dose for phentermine is 8 mg taken 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’ve been prescribed, you should try to 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.

Get weight loss results

Alternatives to phentermine

Many weight loss drugs, including phentermine, work by suppressing your appetite. Though phentermine is the most popular prescription only weight loss 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 others, and compared their similarities and differences to phentermine.


Phendimetrazine 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. Brand names for phendimetrazine include Adipost, Bontril PDM and Plegine.


Methamphetamine is a powerful central nervous system stimulant sometimes prescribed to treat obesity. However, because of how powerful and addictive it is, it will only be recommended as a last resort, after other methods and medicines have failed. It’s sold under the brand name Desoxyn.


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 breastfeeding. Benzphetamine is marketed under the names Rigimex, Didrex and Recede.


Diethylpropion is a stimulant and, like phentermine, works as an appetite suppressant. However, clinical studies have shown that phentermine is more effective as a weight loss aid than diethylpropion. Diethylpropion is sold under brand names Tenuate, Tenuate Dospan and Tepanil.


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.


Unlike phentermine, 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. Because of this, it’s suitable for long term use. Orlistat is sold under the brand names Alli and Xenical.

The natural, prescription-free alternative

Now that you know a little more about phentermine, you may be having second thoughts about taking such a drastic (and risky) step to lose weight. Don’t worry, that’s perfectly normal – and you’re not alone.

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

Just like phentermine, PhenQ works best when it’s taken alongside a healthy lifestyle. Dieters from all over the world have found it to be a highly effective (and more affordable) alternative to phentermine, with some even claiming they’ve had to stop taking it as they’ve lost too much weight!

PhenQ today without prescription and see what you could achieve! Click here >

Pill for a dream body



  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.