How Long Does Phentermine Stay In Your System

With obesity continuously on the rise, more people are trying different things in order to lose or control weight. This includes taking medications intended to aid in weight loss or to suppress hunger. Phentermine is one of these drugs that is popular in the weight loss community. This is usually prescribed by a doctor and is known under the brand name Fastin or Adipex. It was originally made to replace amphetamines and should always be used with caution and according to your doctor’s instructions.

Here, you’ll find some useful information about the diet pill including an answer to the commonly asked question, “how long does phentermine stay in your system?” All of the information will hopefully help you learn more about the drug and determine if it is right for you.


What Is Phentermine?

Classified as an anorectic, Phentermine is a weight loss pill that is currently one of the most popular prescription medications on the market. It is meant to be used in conjunction with diet and exercise in order to lose weight. While it does seem to work in most cases, the reason why it works is less clear. Some believe that phentermine decreases your appetite while others believe it increases energy levels which help you be more active. Still, others believe that it may affect different parts of your brain to help control hunger and to activate your body to burn calories better. In general, it is labeled as an appetite suppressant, though.

​Usually, phentermine is prescribed to take once a day. Your doctor will give you specific instruction that should be followed properly. It is taken by mouth either one hour before breakfast or one or two hours after eating breakfast. In some cases, a doctor might suggest taking up to three small doses a day. One thing to keep in mind though is that taking this medication too late in the day as it tends to cause insomnia in some people. The regular tablets should be taken whole with a full glass of water and should not be crushed up. If you are taking the dissolvable tablet, place it on top of your tongue until it fully dissolves then swallow the remnants with or without water.

​This shouldn’t be taken with other appetite suppressants or for extended periods of time. A doctor will usually take you off phentermine after a few weeks to avoid any complications. You will most likely have to be taken off the medication gradually in order to avoid withdrawal symptoms. If you stop the medication suddenly you may experience fatigue, depression, and headaches. Any symptoms like this should be reported to your doctor to avoid further problems. While there is no widespread concern about addiction, there is a slight chance that it can cause dependency. Do not increase the dose or the frequency on your own. If signs of an addiction are apparent, your doctor may take you off the medication.

There are some side effects that can be expected, as there are with any prescription medication. Some of these effects are:

  • Diarrhea or constipation
  • Irritability
  • Dizziness
  • Nausea and vomiting
  • Insomnia or trouble sleeping
  • Dry mouth

While these are common side effects, they should lessen over time or might not be experienced at all by some patients. If they remain constant or worsen, talk to your doctor and see if the dose should be changed or if you should stop taking the medication completely. You may also be asked to monitor your blood pressure regularly as phentermine may raise your blood pressure. More serious side effects which are rare and should be reported immediately are:

  • Irregular or rapid heartbeat
  • Uncontrollable/severe mood swings
  • Severe headache
  • Seizures
  • Weakness in one side of the body
  • Involuntary muscle movements
  • Increased or decreased libido
  • Slurred speech
  • Blurred vision
  • Chest pain
  • Fainting

Before you take phentermine, keep the precautions in mind. It shouldn’t be taken with other sympathomimetic amines. This includes pseudoephedrine, amphetamines, and diethylpropion. You should also avoid this medication if you have certain medical conditions including cardiovascular problems, history of drug/alcohol abuse, glaucoma, diagnosed mental health problems, pulmonary hypertension, history of strokes, or hyperthyroidism. Your doctor will go over your medical history with you and determine how safe phentermine can be for you.

You shouldn’t drive or use heavy machinery until you see how the medication affects you. Once you see that the drug doesn’t cause problems like blurred vision or dizziness with you, you will be able to resume your normal behavior

How Long Does Phentermine Stay In Your System

As with any medication, in order to answer the question “how long does phentermine stay in your system?” you have to examine the half-life of the medication and then take different factors into account that may alter the time it takes for phentermine to clear your system.

In general, the half-life of phentermine is anywhere from 19 to 24 hours. Based on this it is calculated that the drug is detectable in your system for five to six days after the last dose is taken. Standard drug tests have detected phentermine in the system during this time period as well.

This timeframe has more to do with the patient in question, though. Your height and weight can affect this as can outside influences. Some of these variables are:

  • Caffeine: ingesting caffeine has been reported to lower the amount of phentermine present in the blood stream. This is because the caffeine pushes the mediation through your body faster so it metabolizes faster. It can also reduce the effectiveness of the drug.
  • Other medications: if you are taking other prescription medications, they can cause phentermine to stay in your system longer. Anti-depressants, herbal remedies, and certain vitamins can alter the timeframe.
  • Individual body type: your height, weight, and general health will determine your metabolic rate (the amount of energy used up by your body in a certain time period). If you have a high metabolic rate, phentermine can move through your system faster than usual.
  • Dosage and usage: the larger the dose and the longer you take the drug can determine how long the medication stays in your system. If you have been taking the medication for a while and at a high dose, it can take longer to clear your system. Similarly, the drug comes in immediate release and a time-released form. The latter may last longer in the body.


Hopefully, the information here has taught your more about this diet pill and has answered your question of “how long does phentermine stay in your system?” Please remember to always follow your doctor’s orders and talk to them about any concerns you have. The information here should not be used to go against your doctor’s suggestions/orders. Be sure to report any complications or worrisome side effects, too.

Anna Smith

Hi there ! I’m Anna Smith, chief editor at Healthankering. I'm a proud mother of three passionate about health tips, beauty and ways to live healthier with more energy ! We start Healthankering to provide advanced material about not only the best ways to get healthy, but also to entertain and create a great community. Welcome aboard !

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