Phentermine Withdrawal Guide (symptoms, timeline, fatigue)

January 9, 2024 |

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Phentermine withdrawal can cause some side effects because it alters the state of neurotransmitters in your brain.

Phentermine is an FDA-controlled stimulant because it has the potential for abuse, although it might not be as high as amphetamines.

Phentermine artificially stimulates the sympathetic nervous system to increase the level of certain neurotransmitters that cause an increase in energy and suppress appetite, so when this stimulation does not occur anymore, it may lead to some side effects.

In this article, I will discuss some commonly reported phentermine withdrawal side effects, why they occur, how long they last, and how you can deal with them.

Furthermore, I will also fill you in on the best phentermine alternative available out there.

So, keep reading to find out all these and more.

Phentermine withdrawal symptoms

Phentermine cessation may cause some physical or psychological effects on the user. This drug has been around since the 1950s and is also one of the most prescribed, tested, and trusted weight loss pills.

It secured an FDA approval for short-term use in 1959. And in 2019, it was ranked as the 158th most prescribed drug in the United States, with over 3 million prescriptions around the country.

Regardless of all these benefits of the drug, there is one small snag attached to it, and it may be ascribed to its similarity with amphetamine and to its being a central nervous system stimulant.

Both of these phentermine traits hint at its potential for abuse, and these are the same reasons why it is classified as a Schedule IV controlled substance.

Nonetheless, phentermine is not a speed drug, and its potential for abuse is relatively low compared to amphetamine, a Schedule II stimulant.

Moreover, a study to inspect the withdrawal symptoms of phentermine was performed on thirty-five obese patients who were enrolled in a weight management program but abruptly discontinued their prescription.

According to the reported results, abrupt cessation of phentermine only causes a loss of therapeutic effects of the drug instead of causing any withdrawal symptoms.

Thus, this may hint at the fact that even if therapeutic phentermine does have any addiction potential, it must be of little consequence.

The results also mentioned that ‘craving’ is one of the hallmark characteristics identifying substance dependence and abuse.

However, not a single participant in the study reported this side effect from the abrupt cessation of phentermine.

Moreover, there were no significant differences between the states of patients who abruptly discontinued the phentermine drug and others who never took it.

However, there were striking differences between the conditions of phentermine-treated and amphetamine-treated patients.

Another study recruited 269 obese patients to check phentermine’s addiction potential, and the results were later published in the Nature journal.

The researchers found that phentermine does not cause any withdrawal craving, psychological dependence, and drug abuse when patients take it to treat obesity.

And amphetamine-like withdrawal symptoms don’t occur in phentermine-treated patients, neither when they abruptly stop taking it nor when treated with a higher than the recommended dosage (112.5mg) or for up to 21.5 years.

The summary of the studies I presented to you in the above paragraph might have convinced you that phentermine is not reported to cause any adverse withdrawal symptoms.

Nonetheless, you might feel some side effects from giving up on your prescription.

And according to the user experience, Jenn2222 wrote on the forum medhelp that she took phentermine for about a year and has been off the drug since the last month.

But she is still facing phentermine withdrawal, anger, depression, panic attacks, and insomnia.

Potential phentermine side effects after quitting

So, below I’ve stated some common side effects that might come into effect after you quit using phentermine.

1. Low energy levels

Phentermine stimulates the sympathetic nervous system, which prepares your body for danger by regulating the release of energy from glucose and fatty acids.

It makes energy available to the body so that you are prepared to throw a punch if the need arises. You may feel this phentermine energy for 10 to 14 hours.

The chronic use of this stimulant drug may downregulate the natural supply and release of dopamine and catecholamines that normally regulate your energy levels.

And thus, once you are off the prescription, phentermine withdrawal fatigue may kick in after a few hours because your body has run out of artificial stimulation that triggers the release of these neurotransmitters.

2. Hyperphagia

Phentermine belongs to the class of drugs called anorectics, and as the name implies, it works to reduce your appetite so that you eat less and lose more.

But after you quit your prescription, your desire to eat and your carbohydrate craving may return since the appetite suppression caused by phentermine is no longer in action.

Thus, ‘hyperphagia’ is one of the most commonly reported phentermine withdrawal effects, and this term refers to binge eating or overeating.

3. Low activity level

As you now have an idea that a low energy level is most probably going to be the first withdrawal symptom that you may face while trying to wean off phentermine, and because of it, low activity levels may follow.

Another reason demotivating you to move around might be the regain of fat mass caused by hyperphagia, and I have already discussed hyperphagia in detail.

4. Mood changes

Since by discontinuing phentermine, you are letting go of a substance that is artificially triggering your brain stimulation, so it may take some time for your brain to stabilize the neurotransmitter release and take it back to normal.

Hence you may feel changes in your mood as one of the withdrawal symptoms of phentermine.

Some commonly reported psychological effects are mood swings, tension, irritability, depression, anxiety, paranoia, and emotional flatlining.

5. Hypersomnia

Hypersomnia is a common drug withdrawal symptom – the opposite of insomnia; an increased feeling of sleepiness.

By now, you might be well aware of phentermine’s effect on your neurotransmitters and energy levels and what role they might play in causing the phentermine withdrawal effects. Similarly, hypersomnia may also be attributed to depleted energy levels and low levels of neurotransmitters like dopamine and norepinephrine.

Multiple studies like this recent 2020 study have reported that dopamine promotes wakefulness.

And another one of these discoveries, it was reported that dopamine inhibits the production and release of melatonin by affecting different biological processes and intermediate neurotransmitters. (Melatonin is a neurotransmitter that prepares the body to sleep and causes sleepiness.)

On the other hand, as mentioned above, phentermine withdrawal may also cause low levels of norepinephrine.

And according to a scientific paper, norepinephrine promotes alertness and attentiveness. It also blocks melanin production to keep you from yawning the whole day.

There are no direct studies performed to justify and explain the link of hypersomnia as a phentermine withdrawal symptom. S

o, I have developed these hypotheses on the basis of my personal reasoning and the studies I could find.

Thus, development in research might explain the link better, so you should not regard the explanation provided in this section as full and final.

6. Inattentiveness

Again, both dopamine and norepinephrine increase attentiveness and alertness. Thus, a decrease in these neurotransmitters in the brain caused by withdrawing the stimulant phentermine may cause inattentiveness.

Norepinephrine is also involved in focus and memory retention, which may also help to keep you hooked to your tasks.

7. Gastrointestinal Problems and other symptoms

Some users have also reported gastrointestinal symptoms such as nausea, abdominal pain, diarrhea, stomach cramps, constipation, etc., following phentermine discontinuation.

And others have stated that discontinuing phentermine causes a headache or lightheadedness.

These symptoms might be linked to your psychological states like anxiety or depression. However, due to the lack of significant research on phentermine withdrawal symptoms, I can’t provide you with evidence as to why it might be so.

How long do phentermine withdrawal symptoms last?

Phentermine withdrawal symptoms may last anywhere from a few days to a couple of months.

The type and severity of the side effects caused by withdrawing from any drug are increasingly dependent on its administration route, dose, and duration of the prescription.

The reason is that drug administration through inhalation, and intravenous route might cause a high plasma concentration of the drug.

In contrast, a higher dose of the drug or taking it for a longer duration may cause a large buildup of it in the body.

According to the standard prescription of phentermine, it is supposed to be taken via the oral route and for a short-term duration of 12 weeks.

Taking the drug via mouth prevents an abrupt spike in the plasma drug levels and reduces the risk of causing serious side effects and withdrawal symptoms.

According to, withdrawal symptoms vary for everyone, but they are most acute during the first four days and then start to clear up. However, symptoms like low energy levels and depression may last for a few months.

And according to the phentermine withdrawal timeline stated on, the withdrawal symptoms may occur within 6 hours of discontinuing the last dose.

And the average time of onset of the physical withdrawal symptoms is within 36 hours of drug cessation.

The symptoms may peak during the first six to seven days of quitting the prescription and may start to taper off after that gradually.

How to deal with phentermine withdrawal?

Before I tell you how you can deal with phentermine withdrawal, you need to understand that the stimulant effect of phentermine triggers a higher production of dopamine and norepinephrine in the brain.

And abruptly terminating the prescription may cause a withdrawal effect because your brain is readjusting to its natural levels of neurotransmitters.

Now, check below to know about ways to help you deal with phentermine withdrawal:

1. Cycle off safely

Your doctor may decrease your dose slowly when putting you off phentermine.

Tapering the drug dosage in the final weeks of treatment helps the brain adjust to the decrease in stimulation and the changing level of neurotransmitters which may prevent phentermine withdrawal effects.

2. Stay hydrated and exercise

Doctors already recommend staying hydrated and physically active when they put you on phentermine. It helps to enhance your weight loss progress on the drug and prevents its side effects like drug tolerance and dry mouth.

Incorporating these two recommendations into your phentermine-free routine might also help you cope with the side effects of discounting the drug.

Water helps to flush out the toxins and acts as an appetite suppressant. Thus, it may help prevent the withdrawal side effects like carbohydrate craving and hyperphagia.

So, it would be best if you strived to complete the daily requirement of drinking at least eight glasses of water every day.

Exercise increases serotonin and decreases the appetite hormones like ghrelin. On the one hand, increased serotonin levels might help counter the psychological side effects like anxiety and depression.

And on the other, a low level of appetite hormones may help prevent post phentermine binge eating in patients by decreasing hunger pangs and cravings.

Moreover, exercise also helps regulate your sleep cycle, which may help counter sleepiness, which may also impact your attentiveness.

However, different types of exercise, such as resistance and aerobic exercise, affect your sleep differently, so you may opt for the one that can relieve hypersomnia.

And as mentioned in this article on, you may try to or build a routine to do aerobic exercise daily such as dancing, swimming, or playing tennis.

3. Shift to a Phentermine alternative

Shifting to an over-the-counter stimulant-free diet pill that does not have any addiction potential may help keep off physical and psychological side effects of discontinuing phentermine, such as hyperphagia, fatigue, and low mood.

These OTC weight loss pills help suppress your appetite, increase energy levels, and uplift your mood. PhenQ might be one of the best OTC phentermine alternatives, and in the coming sections of this article, you can find its details.

Phentermine withdrawal Reddit

The user experience that I found on Reddit coincides with the scientific evidence that I have discussed in the above section.

And according to both of them, phentermine does not have addiction potential, and neither does it cause severe withdrawal symptoms if consumed according to the prescription and standard recommendation of the doctor.

Nonetheless, I dug out this Reddit story in which the Reddit user ‘Jooseyjintshire’ had been taking phentermine for up to two years and is now facing difficulty in weaning off from it.

According to the user, the doctor had prescribed her a half pill in the morning instead of a full one to taper the drug.

But the first symptom she faced was extreme exhaustion. The second was an excruciating headache behind her right eye and stabbing pain that made her nauseous.

Safer Alternative to Phentermine without withdrawal symptoms

As I mentioned above, opting for over-the-counter diet pills that don’t have stimulants may help you deal with phentermine withdrawal symptoms.

But how about making them your first choice to aid your weight loss?

Many safer alternatives to phentermine have entered the markets, such as Trimtone, Phen gold, Phen24, and PhenQ, that won’t put you at risk of phentermine withdrawal side effects.

But PhenQ is my personal favorite because of the four main reasons below:

  • It is manufactured by Wolfson Berg, which is one of the leading and trusted names in the weight loss industry and has been around for more than 15 years.
  • It has a weight loss success rate of about 95%, and the company also claims that it has helped more than 190,000 users lose weight.
  • The third reason is that the PhenQ pills are formulated with a 5-in-1 formula that not only suppresses appetite but also burns fat, prevents fat accumulation, boosts energy, and enhances mood.
  • Last but not least, it is free from phentermine and is based on scientifically-backed all-natural and vegan ingredients that include vitamins, minerals, and fiber, which do not cause serious side effects and especially any withdrawal symptoms.

A bottle of PhenQ has 60 tablets and costs around $70, and the company assures you a 60-day money-back guarantee if you are not satisfied with the results.

As it is not a prescription drug so, you can easily order this safe alternative to phentermine directly from the original website right here.


To conclude, phentermine withdrawal effects are not as severe as amphetamine withdrawal symptoms.

And these effects are most commonly characterized as a loss of therapeutic effect of phentermine rather than withdrawal symptoms.

These side effects include exhaustion, increased appetite, low activity levels, increased sleepiness, low mood, lack of attentiveness, headache, gastrointestinal symptoms, etc.

Nonetheless, it is best to slowly decrease your phentermine dosage to get off of phentermine safely.

And remember to closely follow your doctor’s prescription because taking a higher than the standard dosage for a longer than a standard duration may make it harder for you to withdraw from the drug.

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