Phentermine Stopped Working? Let’s Explore Solutions:

by Rakib Sarwar, RPh
Published on December 19, 2018 and last updated for accuracy on September 19, 2022
blue and white pills, illustrating Phentermine

If you have used Phentermine once, you have or will probably use it a second time too. We are not saying that Phentermine causes “yo-yo weight loss,” but a lot of users get back to it. Some of them say that Phentermine stopped working. Is that true?  

Frustration if your savior weight loss medicine doesn’t work anymore is reasonable.

There are some tips on how to increase its effectiveness and how to get rid of the excess weight for good. Many other factors also affect the results and its activity.

Our body learns, gets used to different natural and synthetic substances, and develops a particular form of self-defense. Something similar happens with the repeated use of Phentermine. Luckily, there is something we can do about it.

Let’s first learn more about this medicine.

What is Phentermine?

Phentermine is a popular weight-loss medication. Its scientific, full name is “phenyl-tertiary-butylamine” (1). As its name implies, it is based on a compound called “phentermine”.

All weight loss pills are created with the same purpose – to help us in the weight loss process. However, different pills operate in different ways.

Some of them reduce appetite; others accelerate metabolism, the third group blocks carbohydrate or fat absorption. Many weight loss pills combine several of these effects at the same time.

The active ingredient of this medicine is phentermine hydrochloride, followed by the number of inactive ingredients.

This medicine comes in two forms:

  • Capsules
  • Tablets

Capsules contain the following auxiliary and inactive ingredients:

  • cornstarch,
  • gelatine,
  • lactose monohydrate,
  • magnesium stearate,
  • titanium dioxide,
  • black iron oxide,
  • Blue 1, Red 40, and Red 33 (2).

The tablets have various inactive ingredients, including:

  • cornstarch,
  • lactose,
  • magnesium stearate,
  • microcrystalline cellulose,
  • pregelatinized starch,
  • Sucrose,
  • Blue 1 (2).

Both tablets and capsules have the same effect.

The only difference is the fact that the capsules have the extended-release of active substances. Your doctor will decide what the best option for you is.

How does it work?

Phentermine’s fundamental principle is based on reducing the appetite (2). It is known that a poorly controlled appetite causes you to gain weight. Phentermine affects the hypothalamus and the adrenal gland. It stimulates the release of neurotransmitter norepinephrine that regulates many physical processes in our body and brain. Namely, it leads to increased metabolism, increased glucose utilization, higher lipolysis, and using fat as an energy source.

Above all, phentermine reduces the feeling of hunger and creates a sense of satiety (3).

Physiological Effects of Phentermine

This group of drugs acts primarily on the neurotransmitters of the central nervous system. Many neurotransmitters participate in the regulation of food intake, feeling of satiety, binding of fat to the receptors, triggering processes that result in hunger and increased consumption of food or by decreasing appetite and consequent reduction of food intake (4).

This may all sound a little bit confusing. Let’s explain in depth how it works.

Medications for obesity treatment imitate or induce neurotransmitters that cause feelings of satiety or inhibit those neurotransmitters that cause hunger. This is the mechanism of the noradrenergic drugs, serotoninergic drugs, and those that bind to the receptors for the gamma-amino-lactic acid or cannabinoid receptors, and some peptides that reduce appetite or stimulate feelings of satiety (3).

Noradrenergic drugs stimulate the release of noradrenaline or block its re-storage in neurons, affecting the natural process of food intake. Noradrenaline is released from storage in the presynaptic vesicles and binding to beta-adrenergic receptors in the peripheral hypothalamus decreases feed intake (5).

Phentermine and diethylpropion are sympathomimetic amines that resemble amphetamine. However, unlike amphetamine, they do not have or have only minimal effect on the release of dopamine in the synapse (5).

One of the interesting features users notice after several series of taking Phentermine is that it “stopped working”. Now that we know more about the mechanisms of this medicine, let’s see why its effects seem to be weaker the second, third, or fourth time and what we can do to make it as effective as the first time. We will help you learn how to use Phentermine for the last time, i.e., how to maintain the desired weight.

Phentermine Stopped Working? Potential Reasons:

Everything we learned about Phentermine so far doesn’t sound bad at all. However, what is the problem with this medicine? Why do the patients experience weight gain recidivism several times? What are the factors that interact with Phentermine? Why do so many people complain that phentermine stopped working? Maybe you did lose weight at first but now find yourself experiencing a high Phentermine tolerance. 

Phentermine tablets and capsules are very efficient. However, they are suitable for short-term use (6). The pill will not do the hard work for you. It will help you go through some hard experiences during your weight loss struggle that makes many people give up. Namely, we are speaking of the starvation crisis, anxiety, etc. Lots of people give in their urge for food, urge for carbohydrates or eating late, during the night. These are the main factors to look at when asking will Phentermine works, and why it works.

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Phentermine Needs A Complete Program

Patients taking Phentermine will not have to struggle with these issues. However, the rest of the hard work is on you. You must keep a balanced diet after your phentermine therapy is over to prevent gaining weight. If you continue to consume unhealthy foods and do not exercise, then the diet pills will not provide you with effective results. 

Furthermore, it is not Phentermine that stopped working; it is the fact that your body is intelligent. Many people do not realize that Phentermine tolerance is a possibility. It learns the patterns of our behavior and fights us back. After several series of gaining and losing weight, with or without phentermine, it is very likely that you will not be able to control your weight loss that easily anymore. You will have to change your habits.

Let’s take a look at some useful pieces of advice when it comes to weight loss with this medicine.

Taking a break between two Phentermine treatments

Taking a break between two treatments or sessions with phentermine is advised by most doctors.

This is very important, as phentermine was made for short-term use (6). It is a must to establish a natural and healthy eating routine that will provide all necessary nutrients to our body.

Should I use vitamin B-12 with Phentermine?

Phentermine is a weight loss medication that does not interact with vitamin B-12 and has no pharmacological connection to the nutrients.

However, experts explain that weight loss clinics sometimes include the B-12 supplement in their Phentermine programs, claiming that this vitamin significantly improves metabolism (7). However, there is little scientific evidence supporting this claim.

If your doctor prescribes vitamin B-12 while you are on phentermine, you probably have a lack of nutrients which is easily detected via laboratory tests. Since the weight reduction drug suppresses appetite, you may not get the right amount of vitamins. Prevent the B-12 deficiency by following your doctor’s dietary recommendations when you start with phentermine therapy.

Also, if you follow a vegan diet and are using phentermine, you will need a vitamin substitute. Low levels of vitamin cause anemia, as well as tiredness and neurological problems.

Phentermine dosage and use

You must always use Phentermine only with the prescription from your doctor. Beware of fake drugs with the same or similar name sold illegally, without the medicine.

Always stick to the dose prescribed by your doctor. You can get either the extended-release capsules or the tablets. You must never take more Phentermine than prescribed.

Furthermore, make sure you stick to the program made by your doctor. Most specialists will prescribe phentermine only for 3 to 6 weeks (8). After this period, you should take a break.

Always report all medical conditions and drugs you are using to your medical care specialist before starting the Phentermine therapy.

If Phentermine stopped working, report this to your doctor. Some people may feel that they want to increase their dosage. This, however, should never be done without a doctor’s instructions.

A high dose of Phentermine can lead to more severe side effects. If you experience negative effects, especially after a higher dose, it is important to contact your doctor immediately.

The role of mindset in weight loss supported with Phentermine

If you expect that Phentermine will take away your excess pounds like a magic trick, you should either give up on Phentermine treatment or change the way you think about weight loss. First of all, there is no magic when it comes to losing weight.

Without a complete weight loss strategy, Phentermine will be ineffective. Perhaps you will lose weight in the beginning, but you’ll soon end up with questions about why Phentermine stopped working. 

You will have to be devoted to the new lifestyle, eating routine, habits, exercising, even sleeping routine (9). Even if some medicines, such as Phentermine are very efficient, this doesn’t mean that you can get back to your old habits and expect not to gain weight.

Most people repeat Phentermine treatment several times because they don’t stick to the doctor’s tips on healthy and balanced nutrition supported by regular exercise. In this case, it becomes more likely to reach a point where Phentermine stopped working.

Your weight loss progress comes to a halt, even if you continue using these prescription drugs. 

Work on your mindset. There are methods that you can use to avoid a situation where Phentermine stopped working due to unrealistic expectations. One effective strategy is to ensure you understand exactly how the drug works.

Phentermine works through appetite suppression. Learn what this means, then determine how you can use it to effectively lose weight. 

Tips and tricks on staying fit with Phentermine

We want to share some useful tips and tricks on staying fit with Phentermine. If you are struggling with weight loss, you have probably heard of these before. However, it is important to emphasize their importance, as it is your lifestyle that helps you keep your weight after Phentermine enables you to lose those excess pounds.

  • Take Phentermine for a short period (3 – 6 weeks)
  • Understand it as a trigger for weight-loss, not a constant aid
  • Change your eating habits
  • Start exercising
  • Drink eight glasses of water a day
  • Believe in your success
  • Establish a regular sleeping routine
  • Stay away from the stress
  • Take vitamins.

Final Words

In conclusion, you should benefit from Phentermine as a boost for your weight loss, as a tool, not the purpose. Phentermine will help you quickly get used to eating less and exercising more (8). However, the rest of the work is on you.


(1) Group of authors. Phentermine. Pure Chromatography. Published online on: n.d. Found online at:

(2) Author: unknown. Phentermine Hydrochloride – phentermine hydrochloride tablet. Direct RX. Published online on: n.d. Found online at:

(3) Klein S, Romijn J A. Obesity in Williams Textbook of Endocrinology (Thirteenth Edition). Published online on: 2016. Found online at:

(4) Berthoud H. (2002) Multiple neural systems controlling food intake and body weight. NeurosciBiobehav Rev 26: 393–428 [PubMed]

(5) Bray GA. Treatment of obesity with drugs in the new millennium. In: Eckel RH, editor. Obesity: mechanisms and clinical management. Philadelphia: Lippincott Williams & Wilkins; 2003. pp. 449–475.)

(6) Nicolai J P, Lupiani J H, Wolf A J. An Integrative Approach to Obesity in Integrative Medicine, Chapter 37. 4th edition. Published online on 2018. Found online at:

(7) Michael Lang. The Effect of Phentermine and B12 on Weight Loss Among Obese Patients. Published online on: Aug 13 2015. Found online at:

(8) Kim KK, Cho HJ, Kang HC, Youn BB, Lee KR. Effects on Weight Reduction and Safety of Short-Term Phentermine Administration in Korean Obese People. Yonsei Med J. 2006 Oct;47(5):614-625. Found online on:

(9) Cespedes A. Tips on Losing Weight on Phentermine. Published online on: JULY 18, 2017. FOUND ONLINE AT:

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