How Long Does Phentermine Stay In Your System? | Phentermine Half-Life

Phentermine is an FDA-approved prescription medication primarily used as an appetite suppressant to aid in weight loss. More than 2 million prescriptions of Phentermine were recorded in 2021, making it one of the most widely used weight loss medicines. Phentermine is typically prescribed for short-term use in combination with a doctor-approved, reduced-calorie diet, exercise, and behavior change program to help treat obesity.

If you’re taking or considering phentermine, understanding how long it remains in your system is crucial, whether it’s for drug testing, managing side effects, or other medical considerations.

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Phentermine is a powerful appetite suppressant and stimulant that belongs to a class of drugs known as sympathomimetic amines. It is typically prescribed for short-term use to help obese individuals lose weight when combined with diet, exercise, and behavior modification.

The drug works by stimulating the central nervous system, by increasing the level of certain chemicals in the brain. These chemicals act as messengers that reduce hunger signals in the brain, making people feel less hungry. Additionally, the increase in norepinephrine also leads of elevation in energy levels. This energy boost helps people feel more active and potentially enhances their physical activity levels, further contributing to weight loss.

Is Phentermine A Controlled Substance?

Yes, phentermine is classified as a Schedule IV controlled substance under the Controlled Substances Act in the United States. This classification indicates that phentermine has a potential for abuse and dependency, although it has a lower risk compared to Schedule III substances. Because of its controlled status, phentermine is only available through a prescription and should be used under strict medical supervision.

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Phentermine Half-Life

The concept of a drug’s half-life is essential to grasp when discussing how long it stays in your system. The half-life is the time it takes for the concentration of the drug in the bloodstream to reduce by half.

The half-life of phentermine ranges from approximately 20 hours. This means it takes about 20 hours for the body to eliminate half of the dose from the bloodstream. Complete elimination typically requires around 5-6 half-lives. This suggests that phentermine can stay in the system for about 4-6 days, depending on individual metabolic factors.

Understanding how long phentermine stays in your system has several practical implications:

  • Drug Testing: If you’re undergoing drug testing for employment, sports, or medical reasons, it’s essential to know when phentermine will be out of your system.
  • Managing Side Effects: Knowing the duration of phentermine in your system can help manage and anticipate side effects.
  • Safety Considerations: If you’re planning on stopping phentermine or switching to a different medication, understanding its elimination timeline can help avoid potential drug interactions and ensure a smooth transition.

Factors Influencing Phentermine Duration


People with a faster metabolism will process and eliminate phentermine more quickly than those with a slower metabolism. Factors like physical activity, overall health, and genetic predisposition can influence metabolic rate.


Older adults tend to have slower metabolisms and reduced kidney and liver function, which can prolong the elimination of phentermine from the body.

Body Mass

People with a higher body mass may retain the drug for a longer period because the drug can be stored in body fat and slowly released into the bloodstream.

Liver and Kidney Function

The liver metabolizes phentermine, and the kidneys are responsible for excreting it. Impaired liver or kidney function can slow the elimination process, causing the drug to stay in the system longer.

Dosage and Duration of Use

Higher doses and longer use of phentermine can lead to an accumulation in the body, extending the time it takes to be eliminated.

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Does Phentermine Show Up on a Drug Test?

Phentermine can show up on drug tests, especially those designed to detect amphetamines. Standard drug screening panels often include tests for amphetamines. Since phentermine has a similar chemical structure, it can lead to a positive test result for amphetamines. It’s essential to inform the testing authority if you are taking phentermine under a doctor’s prescription to avoid potential misunderstandings.

Drug Detection Window For Phentermine

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Blood Test

Phentermine can be detected in the blood for up to 24 hours after the last dose. Blood tests are typically used for detecting current impairment or recent use.

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Urine Test

This is the most common testing method. Phentermine can be detected in urine drug tests for 1-4 days after the last dose. For frequent users or those on higher doses, it may be detectable for slightly longer.

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Hair Test

Hair follicle tests can detect phentermine for up to 12 weeks or longer. This method is less common for routine drug testing but can provide a longer detection window for chronic use.

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Saliva Test

Phentermine can be detected in saliva for up to 48 hours after the last dose. Saliva tests are less invasive and provide a quick and easy method for detecting recent use.

Withdrawal from phentermine can occur if the medication is discontinued abruptly after prolonged use. These symptoms arise because the body needs time to adjust to the absence of the stimulant effects of phentermine. Gradual tapering off the medication under medical supervision can help minimize withdrawal symptoms.

Common Symptoms of Phentermine Withdrawal

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Increased appetite

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General Withdrawal Timeline

  • Symptoms: The initial withdrawal symptoms can start within a few hours to a day after stopping phentermine. These may include fatigue, increased appetite, irritability, and mood swings.
  • Physical Effects: You might experience a decrease in energy levels and possibly mild depression due to the sudden drop in stimulant effects.
  • Increased Symptoms: Symptoms like fatigue and irritability may persist and can be more pronounced. You might also experience headaches and sleep disturbances.
  • Cravings: There may be strong cravings for phentermine or other stimulants as your body adjusts to its absence.
  • Improvement: Some symptoms, such as irritability and mood swings, may start to improve. However, fatigue and increased appetite can continue.
  • Weight Gain: There might be a noticeable increase in appetite, which can lead to weight gain if not managed carefully.
  • Stabilization: Many withdrawal symptoms start to subside, and your body begins to stabilize without the drug. Energy levels and mood gradually improve.
  • Lifestyle Adjustments: It’s important to focus on healthy eating and exercise habits during this phase to counteract any potential weight gain and maintain overall well-being.

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Always consult your healthcare provider for personalized advice and information regarding phentermine and its use. Being informed about the medication can help you manage its effects and ensure it’s used safely and effectively.

If you or someone you know is struggling with phentermine addiction, seeking professional help is crucial. The Hope House, a distinguished addiction treatment center in Scottsdale, Arizona, provides comprehensive addiction treatment services tailored to individual needs.

Our team of trained specialists offers a compassionate and supportive environment to help individuals overcome addiction and reclaim their lives. Don’t wait to get the help you need; contact The Hope House today to start your journey towards recovery.

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