Phentermine is an FDA-approved weight loss medication for weight loss in adults. Phentermine’s primary mechanism of action is the release of norepinephrine from the brain causing a suppression of appetite, increase in energy, and release and break down of fat. Phentermine works best when used with proper nutrition and adequate physical activity and should be monitored by a doctor.

Phentermine is not an amphetamine but works similarly to well known amphetamines, however, does not cause addiction or physical dependency.

Did phentermine stop working or did you stop working?

why phentermine stopped working

The first step is to determine if your eating habits and level of activity are to blame. Below are a few questions to consider:

  • Are you stalling on the scale but losing body fat, inches, or going down in clothing sizes?
  • Are you taking phentermine daily first thing in the morning?
  • Are you drinking enough water?
  • Are you following the nutrition recommendations set by your doctor?
  • Are you logging your food intake to uncover hidden carbohydrates, sugar, and/or calories?
  • Are your food portions too large?
  • Are you consistent or do you only follow the proper plan part time?
  • Are you falling back into bad habits from the past?
  • Do you need to increase your physical activity level?


Phentermine is not a magic pill; it is a tool that aids in weight loss when used correctly. If you want to experience lifelong results take phentermine with the expectation to change daily habits and food choices. As you continue to make these changes consistently you will transform your physiology and create a new weight set point in the brain that supports a lower weight.

Why does phentermine wear off?

The effects of phentermine are diminished over time due to drug tolerance. Tolerance is when a medication loses potency over time after repeated use and as a result the individual experiences less response from the drug.

Although phentermine is not “addictive” it results in tolerance over time. Tolerance can be good and bad and occurs due to the body’s attempt to create homeostasis (or a predictable balance). In the case of phentermine, reduction in thirst and insomnia can be beneficial; however, losing appetite suppression and energy is less desired.

How it happens?

Phentermine binds to receptors in the brain and releases norepinephrine. The norepinephrine results in weight loss, however, over time these receptors downregulate (reduce in number) so that less norepinephrine is released from the brain over time. The overall impact is the individual taking the medication notices the results are less potent than before.

How will I know if phentermine stopped working?

  • More consistent hunger
  • Able to eat larger portions than previously
  • Need more food to feel satiated
  • Cravings that were better controlled return
  • Prolonged weight plateau (2-4 weeks)

How long will phentermine last? 

The amount of time that phentermine lasts varies based on the individual. Some people have no effect from phentermine, others notice the original impact wears off quickly while others will experience the same effect for several weeks to several months.


What should I do if phentermine stops working?


#1. Don’t freak outw

Don’t give up on your weight loss goals. Don’t give up!

Resist the urge to entertain negative thoughts regarding your ability to successfully lose weight, this will only make matters worse.

The first thing to do is RELAX. If phentermine stops working it is not the end of the world.

If you followed a proper routine for weight loss, diet, and exercise- you will likely maintain the weight that you’ve lost and even lose more. On the contrary, if you skipped multiple meals, ate unhealthy foods, and made no efforts to make sustainable healthy changes, you may regain all the weight lost and then some (watch Dr Covington’s video “The Most Common Mistakes People make with Phentermine”).

It’s important to avoid the most common mistakes that individuals make when taking phentermine if you want long term weight  loss success. 

In addition to not freaking out, avoid restrictive diets, extreme exercise, or unsafe supplements.

#2 Talk to your Bariatrician

Your weight loss doctor is a great resource to walk you through what to do next. Your doctor may stop phentermine or employ techniques to make the medication more effective.

Check out our Blog article –> How to Choose a Weight Loss Doctor 

#3 Increase physical activity

Exercise releases norepinephrine (similar to phentermine but the effect is much shorter) and may provide a similar appetite suppressing effect and boost in energy. The increased exercise will also counteract any rebound in diet or cravings that you may have. Positive endorphins released from exercise will also help to improve mood so that you can remain optimistic regarding your weight journey.

#4 Take a “drug holiday”

A drug holiday involves intentionally stopping a medication for a period of time with the intention of restarting it at a later date. A drug holiday should be facilitated by your doctor, you don’t want to stop phentermine unnecessarily or incorrectly.

Brain receptors change in number based on the stimulus so when phentermine is removed the norepinephrine receptors up-regulate again (or increase in number). A drug holiday often allows the brain receptors to reset and can improve phentermine effectiveness if it is restarted.

As many frustrated patients have experienced, phentermine can become less effective over time due to drug tolerance. The amount of time for phentermine’s effectiveness varies so maximize your results by making lifestyle changes. If you make lifestyle changes the goal you will experience weight loss results that last a lifetime.

Watch Video “How Phentermine Works

Schedule Appointment with Dr Covington