How Long Does Phentermine Stay in Your System?

Phentermine is a prescription stimulant drug that has been approved by the FDA for treating weight loss in overweight people. It is used in combination with a diet and exercise plan to treat obesity in people who also suffer from diabetes, high cholesterol, or high blood pressure.

Although the effects of phentermine are relatively short-lived, they have the potential to linger in the body for several days. Phentermine can also interact negatively with other medications, supplements, and substances; therefore, it is important to be aware of its potential presence in a user’s system. Because of its chemical structure and its classification as an illicit controlled substance, phentermine can sometimes be flagged by drug tests, including urine, blood, hair, and saliva tests that employers and athletic organizations use for screening.

What is Phentermine?

Phentermine is a prescription weight-loss medication for people over sixteen years of age. It’s an anorectic drug that works by stimulating the central nervous system to release chemicals that decrease your appetite.

In the 1990s, it was combined with fenfluramine and dexfenfluramine to create a treatment commonly known as fen-phen. After reports that this regimen could cause serious heart problems, fen-phen was pulled from the market. However, today, phentermine is manufactured and sold as Adipex-P, Lomaira, and Suprenza. It is available by prescription on its own or in combination with topiramate in a drug called Qsymia.

Phentermine is a stimulant that is chemically similar to amphetamine. Consequently, it is classified as a controlled substance, although it has a relatively low risk of dependence when used as directed. Doctors generally prescribe phentermine to obese patients who have a body mass index (BMI) at or above 30 and weight-related high cholesterol, high blood pressure, or type 2 diabetes.

Factors Influencing How Long Phentermine Stays in Your System

Research presented to the Food and Drug Administration (FDA) suggests that drugs may be distributed and metabolized differently in people with obesity compared to people of normal weight. These differences in processing could cause the drugs to stay in the body longer and, at the same time, be less effective than expected. Mostly, the findings have been noted to apply to fat-soluble drugs, which are metabolized in a characteristic manner by an obese person due to their relatively large volume and proportion of body fat. As the amount of fat tissue in the body increases, the drug’s distribution space also increases, which can potentially lower its serum concentrations, making it less effective while extending its half-life. After the patient stops taking the medication, their fat tissue will continue to release the drug until the body eliminates it, leading to prolonged exposure. Although phentermine is a water-soluble medication, these studies shed light on potential complications that may occur when it’s used with other fat-soluble medications.

Dosage and Frequency: Impact of Amount and Regularity of Phentermine Intake

A variety of factors, including age, health conditions, and concurrent medication use, can affect the distribution, metabolism, and excretion of phentermine. To maintain the appropriate serum levels and reduce the potential for adverse effects, there may be cases when the dose and frequency of administered phentermine need to be adjusted. The duration of a drug’s potency and the time it takes to leave a person’s system are dependent on its realized half-life.

Role of Liver and Kidney Health in Drug Elimination

The liver is the primary organ involved in drug metabolism, while the kidney’s primary function is excretion. If the health of these organs is compromised, drugs have the potential to accumulate in the body in toxic concentrations.

How Age and Metabolic Rate Can Affect Drug Retention

A person’s metabolic enzyme systems change with age. Starting around age 55, blood circulation and CYP450 enzymatic activity slow down significantly, which can make drug metabolization more difficult. Older adults may need a smaller dose of a drug per pound of body weight, compared to younger patients.

How Long Does Phentermine Stay in Your System?

Phentermine’s Half-Life

The term “half-life” describes the estimate of time it takes for the amount or concentration of a drug to be reduced in the body by exactly 50%.
For instance, if a person takes 100mg of a drug with a one-hour half-life, it is estimated that:

  • 50mg will remain in the body after one hour
  • 25mg will remain in the body after two hours
  • 12.5mg will remain in the body after three hours

… and so on.

In actuality, the half-life of a drug in a person’s body can be influenced by many different factors, including its volume of distribution, and its excretion rate or drug clearance. As an example, a drug that has a half-life of a few hours in a healthy person may stay in the body of a person with kidney disease for days. Most drugs lose potency after four or five half-lives, but they may still be detectable in a person’s body as the drug continues to be in their system.

Research shows that when taken orally, phentermine has a 19-24 hour half-life, which implies that it will stay in your system and be detectable in the body for five to six days. The effects of the medication typically last for four to twelve hours.

Detection Times in Different Tests

Depending on the type of drug test administered, there may be different detection levels for phentermine.

Urine Test

Phentermine is generally detectable in urine for 48-72 hours, but it may be detectable for longer in people who metabolize the drug more slowly.

Blood Test

Blood tests that are performed by a medical practitioner can usually reveal the presence of phentermine for up to 24 hours. In some cases, the drug can be detected in blood after one day.

Saliva Test

How long does phentermine stay in your saliva?
A saliva test that uses a swab can usually detect phentermine for 24 to 48 hours following consumption.

Hair Follicle Test

Hair follicle tests can detect phentermine for up to 90 days after dosage. The test is not typically affected by shampoo, dye, or other hair care products.

The Process of Phentermine Metabolism

After a person takes phentermine orally, it is absorbed through their gastrointestinal tract. In most people, the drug reaches its highest concentration in the blood in three to four-and-two-fifths hours. Phentermine is metabolized by the liver. The mechanisms of metabolization are p-hydroxylation, N-hydroxylation, and N-oxidation, after which the drug is conjugated. Only about 6% of the administered dose is metabolized, while the N-oxidized and N-Hydroxylated metabolites represent 5% of the dose. Cytochrome P450 (CYP) 3A4 is the primary metabolizing enzyme, phentermine, while amine oxidase enzymes take part in oxidoreductase activity. 70 to 80% of a phentermine dose is excreted, unchanged, in urine.

Factors That May Prolong Phentermine Retention

There are several factors that have the potential to prolong the retention of phentermine by a person’s body, including:

Drug Interactions

Phentermine has noted interactions with several medications, some of which can increase the serum level of phentermine or slow its excretion rate. Abametapir and Acetazolamide are two examples of these drugs, but there may be others. Studies have also shown that the excretion rate of phentermine may decrease when it is taken with antacids, which can prolong its effects or increase the probability of toxicity.

Lifestyle Factors

A person’s diet, hydration levels, and activity level can affect their metabolism, which can in turn influence the activity of the enzymes that break down phentermine. If an enzyme’s metabolic function increases, a drug can lose potency. If an enzyme’s metabolic function decreases, the drug may be less well-tolerated.

Health Conditions

Health conditions that affect metabolism and excretion, such as advanced heart disease, kidney disease, and chronic liver disorders, can negatively affect the metabolic processing of phentermine.

Withdrawal and Side Effects

Although phentermine is presumed to have a low potential for addiction, the body can still become accustomed to its presence. If a person has been taking phentermine for some time, they may experience symptoms of withdrawal when they stop using it, including:


Phentermine is a stimulant that can make a person feel more alert and energetic. People who stop taking phentermine may feel tired or weak as their brain adapts to the absence of the drug. Patients who take large doses of phentermine for a long period of time will be the most susceptible to these effects. They may also experience some cognitive impairment, which is usually temporary, but may be worsened by stress.

Weight Gain

Phentermine is an appetite suppressant that encourages people who take it to consume fewer calories. Once a person stops taking phentermine, their appetite may increase, and they may be less inclined to exercise. As a result, they may consume more calories, burn them at a slower rate, and gain weight as a consequence.

Changes in Mood

Phentermine works by changing the levels of the neurotransmitters norepinephrine and dopamine in the brain. These chemicals are responsible for mood-regulation, and after a person stops taking phentermine, the brain may experience a chemical imbalance before it can return to its normal state. During this time, people may experience bad moods, sadness, and depression.

Drug Cravings

After a person stops taking phentermine, they commonly experience psychological cravings for the drug, because they miss the good feelings the drug can provide.

Phentermine is a medication that has potential to induce side effects, including:

  • A dry or unpleasant taste in the mouth
  • Gastrointestinal effects such as constipation, diarrhea, or vomiting

Serious side effects may include:

  • high blood pressure
  • heart palpitations
  • dizziness
  • tremors
  • sleeplessness
  • restlessness
  • shortness of breath
  • chest pain
  • edema
  • blurred vision

Patients who experience these side effects should seek medical attention immediately.

Phentermine Can Be an Essential Treatment in the Fight Against Obesity

Phentermine can be a safe tool in the fight against obesity when it is used under a doctor’s care. It is important to consulting with a doctor and understand its potential risks and effects before beginning a treatment regimen.


List of scientific studies, clinical guidelines, and expert opinions referenced in the post.


Published on: 2024-01-12
Updated on: 2024-06-19