What does Phentermine do to the body? (how it works)

January 8, 2024 |

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In this article, I will discuss the different effects Phentermine can have on your body.

So, you have been struggling with your weight management, and your doctor has prescribed a drug with an alien name ‘phentermine’ to aid your weight loss journey?

And now you are surfing the internet to gain awareness about how it will work and affect your bodily functions in order to help you lose weight?

I have got you covered.

Phentermine is a drug used to aid weight loss in obese and overweight individuals. And it does this by messing with the neuronal signaling in your body as it alters responses in your Central Nervous system.

For this read, I have answered all the questions we’re mostly asked by newbies when it comes to the use of Phentermine.

From how it might affect your metabolism to how long it will take to kick in and how you can get it out of your system and so on, I have discussed everything in detail.

So, please keep reading to get it all clarified.

How does Phentermine work?

The Phentermine drug was initially tested and approved by FDA to promote weight loss. But in recent years, it is also anecdotally used to treat Attention Deficit Hyperactivity Disorder (ADHD).

According to ‘Pharmacy Times,’ as Phentermine resembles amphetamine and stimulates the central nervous system, it may reduce impulsiveness and improve attention in ADHD patients. But there is no significant research that may prove this case, and for now, it is used off-label for this purpose.

In contrast, Phentermine is clinically proven to promote weight loss and manage obesity in patients who suffer from comorbidities like diabetes, hypertension, or hyperlipidemia.

If you are above 16 years old and your BMI is above 27 or above 30, your doctor may prescribe this drug for short-term use (about 12 weeks) in combination with some exercise and a weight-loss diet plan.

Phentermine is mostly available under the generic names Adipex-P, Obenix, and Lonamin.

And as mentioned in this drug record, it is available in the pharmacy as 30mg phentermine base tablets or 37.5 mg phentermine hydrochloride tablets.

Your doctor will usually prescribe a dose of 30 to 37.5 mg phentermine tablet once a day, mostly to be consumed in the morning before breakfast. But it should not be consumed near your bedtime or late in the evening because it has a risk of causing insomnia.

According to the National Institute of Health, taking prescription weight management drugs may promote a loss of more than 10% of your starting weight. And Phentermine is known to cause about 5% weight loss at 8 to 12 weeks, as mentioned in the chapter ‘obesity’ of Williams Textbook of Endocrinology.

Phentermine is an indirect sympathomimetic drug that belongs to the class of medications called anorectics. And as mentioned in this review, it works by stimulating the sympathetic nervous system, which causes the release of catecholamines, namely epinephrine, norepinephrine, and dopamine.

This study found that Phentermine has an effect on serotonin as well. Phentermine then inhibits the reuptake of these neurotransmitters from the synaptic terminals, which implies that when a high level of these neurotransmitters persists in your brain, it suppresses your appetite.

Moreover, as mentioned in this review, it is also believed to cause weight loss by increasing resting energy expenditure. The combination of both these effects works as an effective weight loss strategy in obese patients.

How does Phentermine affect metabolism?

Phentermine does not affect metabolism.

Phentermine won’t help speed up your metabolism to metabolize food faster or burn more fat. But instead, it promotes weight loss by making you feel less hungry, and you are basically put on a calorie or energy deficit upon its consumption.

A calorie deficit means that you use up more calories than you are putting in your body, so your body will utilize your fat stores to meet its energy demands, which will help to slash a few pounds off your weight machine.

What effects does Phentermine have on the body?

I have broadly divided the effects of Phentermine into 2 sections to make it easier for you to get a grasp of them. So without any further ado, let’s get right into it.

Positive effects

  • Low abuse potential

Even though the structure of Phentermine is similar to amphetamine, it is not as addictive. Therefore, Phentermine is classified as a Schedule IV drug based on its low abuse potential, whereas amphetamine is listed as a Schedule II drug.

  • Effective weight-loss pill

This comparative analysis found that phentermine-topiramate is the best and most effective weight loss pill among other available options and shows the highest odds of achieving at least 5% weight loss.

CNBC also referenced this study and reported that phentermine-topiramate caused 75% of weight loss in patients, and they lost the most pounds on average as compared to other drugs.

Negative effects

  • Drug tolerance

Phentermine is not addictive, but it is only suitable for short-term consumption, that is, for about 12 weeks. Because individuals may develop drug tolerance to it, and as a consequence consuming the drug at the same dose might not remain as efficacious.

The reason is that Phentermine increases the neurotransmitters in the brain, and, over time, the receptors for them may downregulate themselves, and the body will not respond to them in the same way as before.

  • Temporary weight loss

Phentermine may be effective in reducing weight while you are on prescription, but this weight loss is not sustainable.

Because after you discontinue its use, your appetite can return, and you might continue feasting like before, which will nullify your weight loss progress.

Thus, you may need to look for alternative solutions to help with your weight management.

  • Side effects

Phentermine may cause side effects like anxiety, increased blood pressure, irregular heartbeat, insomnia, dry mouth, headache, gastrointestinal disturbance, or dizziness.

There are also multiple cases that show the potential of Phentermine to induce psychosis in patients.

Such as this case report which documented the occurrence of repeated psychosis in response to phentermine consumption in a 25-year-old female.

How long does it take for Phentermine to get into your system?

Phentermine is well absorbed from the gastrointestinal tract into the body whether taken with food or on an empty stomach.

And as mentioned in this article, its peak concentrations in the body are reached within 6 hours of consumption.

How can you get Phentermine out of your system?

You might want to speed up the process of flushing out Phentermine if your body is showing adverse effects in response to it.

Another reason could be that the structure of Phentermine is similar to amphetamine, and if you want to take a drug test, you might want to rush its elimination process to prevent false positives.

But there is no proven recommended way to clear out your system from Phentermine magically, and you need to consult your doctor for ways to taper the drug.

Although most of the phentermine drug is excreted unchanged, 6% of it involves the cytochrome p450 system in the liver, which metabolizes the drug.

Phentermine undergoes p-hydroxylation, N-oxidation, and N-hydroxylation, followed by conjugation. And the oxidated and hydroxylated metabolites represent 5% of the total metabolized drug.

So, the first thing you need to do to get Phentermine out of your system is to discontinue its use and let your liver do the work.

You may discuss a titration plan with your doctor, and he may help you get off the drug effectively.

I have picked out some approximate elimination durations provided by doctors at Healthtap so that you can prepare yourself beforehand or have an idea about how much time it will take for you to come out clean.

  • According to Kenneth Smith, the clinical effect of Phentermine will be gone within 24 hours, and the blood will be cleared from Phentermine within 60 hours.
  • According to Masoud, Sadighpour phentermine will take 150 hours t0 get out of your system.
  • According to Donald Alves, this drug will appear in the urine drug test for about 14 days.
  • And according to Susan Rhoads, like amphetamine, Phentermine will also stay in your saliva for about three days.

Increasing your fat intake is not proven to accelerate phentermine elimination from the body. And neither did I find evidence to back up the claim that increasing water intake will flush out this drug faster from your body. Still, there is no harm if you want to give these two hacks a shot.

How is Phentermine excreted?

Phentermine is absorbed in the gastrointestinal tract, metabolized in the liver, and eliminated from the body by the kidneys.

The excretion of Phentermine is dependent on the pH, and its mean half-life is approximately 20 hours under normal conditions.

Otherwise, it mostly varies between 19 to 24 hours. In contrast, the elimination half-life is reduced to 7 to 8 hours in acidic urine (pH <5).

According to NIH PubChem, 70% to 80% of the administered phentermine dosage is excreted in the urine as an unchanged drug.

The different pH also determines what percentage of the drug will be excreted in the urine.

In normal conditions, only 48% of phentermine drug is excreted via urine, whereas in acidic urine, 84% of its dose is eliminated.

Final Thoughts

All in all, Phentermine is a medication that stimulates your nervous system to reduce your appetite and promote weight loss.

However, you should not consume it without the doctor’s prescription since some contraindications, and drug interactions must be considered before using it.

For instance, you need to be careful about consuming Phentermine with alcohol, some drugs, insulin, or uncontrolled blood pressure.

So, be especially careful about monitoring your well-being and weight loss progress when taking Phentermine. If you face some unusual side effects or see no improvement in your weight, then let your doctor know so they may determine your next course.

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