Why Am I Not Losing Weight On Phentermine

Phentermine is one of the most popular natural weight-loss medications and has been approved by the US FDA. It's known for its appetite-suppressant properties and is used for short-term weight loss. That means it makes you less hungry throughout the day.

Phentermine has been used for decades in treating obesity and in those who cannot burn their extra fats solely by eating a calorie-deficit diet and working out. 

In one randomized study, about 95% of obese people who took Phentermine for 12 weeks lost over 5% of their body weight, and 60% of the same group lost nearly 10% of their body weight.

Another study in Mexico also showed that 80% of people lost 5% of body weight after taking Phentermine for six months. 

Now, these figures may convince you that Phentermine is really effective at shedding extra pounds. Probably that's the reason you started taking the medicine. But you might wonder, “Why am I not losing weight on Phentermine?” “Is there something else I should do?”

Well, let's find out the answer in this blog. We will discuss how phentermine works and the reasons why you’re not losing weight. We will also equip you with some useful tips. 

What is Phentermine?

Before jumping to the answer, first, you need to know the basics. 

Phentermine, also called Adipex-P, Lomaira, Fastin, or Pro fast, is a prescription medicine used to suppress appetite. It belongs to the class of medication known as anorectics. 

Phentermine is recommended only for adults with a BMI over 30 or a BMI greater than 27 and at least one weight-related condition such as high cholesterol, high blood pressure, or type 2 diabetes. It is usually taken by mouth in the morning once a day. 

The person taking Phentermine may experience some mild side effects like headache, dry mouth, and trouble sleeping. In some cases, it is prescribed in combination with topiramate (Qsymia®) for effective weight loss.

How Does Phentermine Work? 

Being a stimulant, Phentermine works by stimulating your sympathetic nervous system. 

It contains an organic compound called Adipex as an active ingredient. They work to reduce your appetite and increase the number of calories that you burn at your baseline. 

Once you take Phentermine, you'll likely notice an immediate difference in your appetite. Your appetite decreases, and you will crave less food, which makes it easier for you to stay away from unhealthy snacks and in-between meals.

You'll also notice a bounce in your energy levels, which is why you may feel energetic all day. 

After ingestion, the drug reaches its peak concentration in your body in 3 to 4 hours. However, it takes a few weeks to see the results in your weight. 

Besides suppressing your appetite, norepinephrine reduces food-related thoughts, obsessions, and preoccupations. This is why some people using Phentermine may forget to eat and no longer think about eating. 

Norepinephrine's metabolic rise causes an increase in the usage of stored body fat as well as an increase in the intake of calories at rest over the course of 24 hours.

How Much Weight Can You Lose On Phentermine?

In general, Phentermine can help you lose 3% to 7% of your body weight. Experts say that you can expect to lose anywhere from 3 lbs to 5 lbs in a month with regular use of phentermine drugs. Note that it is only approved for short-term use for up to 12 weeks. 

Rebound weight gain may occur after short-term use as tolerance starts to grow. That means you quickly regain weight right after you complete a diet. Phentermine use over the long run does not result in net weight loss. 

Why Am I Not Losing Weight on Phentermine? – Possible Reasons

The first thing to keep in mind is that Phentermine is not a magic pill. It's just a short-term weight loss solution and not meant to be taken for long periods.

Most significantly, the effects of a diet change combined with Phentermine meds tend to be more clear-cut. 

Here are some of the possible reasons why you are not losing weight with Phentermine: 

Reason 1 – You're not taking the dose properly. 

Knowing when and how to take Phentermine is an important part of committing to this weight-loss medication. Usually, it is taken first thing in the morning. It doesn't matter whether you take it on an empty stomach or after a meal. The timing is all that matters. 

This is because Phentermine's side effects include increased energy and focus, which can lead to insomnia or difficulty sleeping. This is where the cycle begins.

When you have difficulties sleeping, it can make you crave more and eat more the next day, therefore preventing you from losing weight.

Reason 2 – You're not eating enough. 

Some people might be tempted to skip dinner or go without breakfast to speed up their weight loss efforts. Some also try intermittent fasting to avoid eating more. But in order to lose weight, you need to eat something. 

Yes, it's true that Phentermine works by suppressing your appetite, but the effects can sometimes be too much, to the point where people have no appetite and struggle to eat anything at all.

It's recommended to eat a minimum of 1200 calories in a day. If you eat less than that, your metabolism will slow down, and as a result, your body will not be able to burn those fats, making weight loss impossible. 

Reason – 3 You're not eating healthy.

Another reason for not losing weight could be that you're not eating healthy snacks and foods. Processed snacks, sugary drinks, bakeries, and fast foods can interact negatively with Phentermine and hinder its effects on weight loss. 

Moreover, if you eat foods that have lots of sugar, saturated fat, preservatives, and alcohol, they may make the pH of your urine more acidic.

This can cause your body to eliminate Phentermine faster, meaning all the benefits of Phentermine medication are going in vain. 

Reason 3 – You're not being physically active. 

Many people forget to exercise or even walk out of laziness. They believe that once they take Phentermine along with foods, the medication will do everything. But that's now how it works. A diet should always be paired with some exercise

Research has proven that exercise is the key to losing weight with Phentermine for good. Cardio and weight lifting are the most effective ways to burn fat faster. 

If you're just starting out or have some injury, you can do some walking for 30 minutes a day and gradually work your way up to some strenuous exercises.

Reason 4 – You're not drinking enough water.

Water acts as a natural appetite suppressant. It can take up space in your stomach, resulting in a feeling of fullness. The more you drink water, the less hungry you feel. 

What’s more, if you don't have enough water in your body, the system associated with weight loss cannot function properly. But if you drink more water, it keeps your body working at its peak. 

So, while on phentermine medication, try to drink at least eight glasses of water per day. Honestly, it's the best way to fill you up and quench your appetite without adding calories. 

How Long Does It Take For Phentermine To Work For Weight Loss? 

Generally, Phentermine starts working in your body within three to four hours, and you may notice the effects in a couple of weeks. 

On average, people taking Phentermine can expect to lose twice as much weight as they would lose with diet exercise alone. 

Tips To Lose Weight Fast When Using Phentermine

So now that you know the reasons for not losing weight, there's nothing to be worried about. We've got some tips to help you shed some extra pounds when taking Phentermine: 

  • Stick to a healthy diet first. Also, eat enough food, up to 1200 calories per day.
  • Never skip meals. Avoid intermittent fasting for some time. 
  • Don't take exercise lightly. Do it every day. 
  • Drink lots and lots of water to optimize Phentermine. 
  • Take the Phentermine in the morning so it doesn't interfere with your sleep.
  • Get enough good quality sleep. Rest is important for your body. 
  • Walk around for 15-20 minutes after eating a meal. 
  • Use natural fat burners like green tea or coffee. 
  • Phentermine can be habit-forming. You should only take it for the advised duration, so it's best to talk with your doctor about it.

If you find Phentermine is not helping you lose weight, meaning there is a lack of appetite suppression or minimal to no weight loss over a reasonable period, you must stop taking the medicine and switch to other alternatives or talk to a healthcare professional. Some of the options include Semaglutide, PhenQ, and PhenGold. 

Final Reflection

Hopefully, you found your answer to “Why am I not losing weight on Phentermine?”. Phentermine is basically a prescribed drug for overweight and obese people.

It should not be used as a substitute for diet if you want to lose weight. Rather, it must be used along with a proper calorie-deficit diet and exercise.

When taking Phentermine, always remember to stay realistic. Never skip a good, proper diet and enough physical activity. Without these lifestyle habits, weight loss medicines will get wasted. 

The last thing you should know is that weight loss isn't a race. It isn't linear, either. Some days will be down, and some will be up.

So, be patient. Figure out what works for you and is sustainable and healthy. If you change your focus to health, you'll find a healthy lifestyle that creates effortless weight loss. 


1. What happens if Phentermine doesn't help you lose weight? 

Ans: If Phentermine doesn't help you lose weight, stop taking it. Also, never take more doses as advised. Ask your doctor or physician to recommend other weight loss alternatives like PhenQ. 

2. How can you lose weight faster while taking Phentermine?

Ans: If you want to lose weight faster with Phentermine, you should stick to a healthy and low-calorie diet, perform cardio and exercises, sleep for 7 hours, and drink plenty of water. You’ll definitely notice your weight loss results in a couple of weeks. 

3. How much weight should you lose a month on Phentermine? 

Ans: Between 5% and 7% of your body weight. Studies show that a woman can expect to lose around 3 to 6 pounds in a month, whereas a man may expect to lose 5 to 9 pounds. 

4. How long does it take to start losing weight on Phentermine? 

Ans: Once you take Phentermine, you'll feel less hungry in the next two to three hours, and your weight will gradually go down. To be specific, it usually takes about a week to start noticing some weight loss effects in your body. 

5. What is stronger than Phentermine?

Ans: Brontil is a stronger prescribed medication than Phentermine for weight loss. Many people have reported positive results. However, it should be taken for shorter periods of time as it may lead to dependence. 

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Oliver Nelson

Oliver Nelson is a New York based Health Specialist Writer who completed his graduation from Syracuse University back in 2015. His writings were published in the top Healthcare brands in the United States.

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