How long does phentermine stay in your system?

The average time phentermine stays in your system is four days

By Wendy Wisner | May 8, 2024 Medically reviewed by Leslie Greenberg, MD

Key takeaways

Phentermine is an FDA-approved appetite suppressant used short-term to aid in weight loss and disease prevention.
Phentermine stays in your system for an average of four days.
The appetite-suppressant effects of phentermine are usually felt within a few hours after ingesting it and for about one to two days.
There is no way to flush phentermine out of your system quickly.
Once you stop taking phentermine, your appetite will return, and you may gain weight if you don’t continue implementing similar healthy eating and lifestyle changes.

Phentermine is approved by the U.S. Food and Drug Administration (FDA) as a weight loss drug. It is usually prescribed to people who are overweight or obese and who also have other metabolic risk factors, such as high blood pressure (hypertension), diabetes, and elevated cholesterol. Phentermine is intended to be used short-term (3-6 weeks) and is meant to be used in conjunction with other weight loss methods and lifestyle changes. Phentermine is sometimes sold under the brand names Adipex-P and Lomaira. It’s often combined with the prescription medication topiramate to aid in weight loss. The combination medication of phentermine and long-acting topiramate is name-brand Qsymia.

If you are taking phentermine, you might wonder: How long does phentermine stay in your system? Although everyone metabolizes phentermine at slightly different rates, phentermine generally stays in your system for an average of four days.

How long does phentermine stay in your system?

After taking phentermine, it stays in your system for about four days, says Katy Dubinsky, Pharm.D, founder and CEO at Vitalize. The half-life of phentermine is about 20 hours, which means that about half of the medication exits your system in about 20 hours, Dubinsky says. “Peak concentrations of phentermine are reached within three to four hours following ingestion,” she explains.

According to Meghan Garcia-Webb, MD, a triple board-certified internal, lifestyle, and obesity medicine physician, “Patients should expect to feel the effects of the phentermine for 24-48 hours, but not much longer afterward. This is because phentermine typically stays in your system for several days, so you won’t necessarily feel its effects the entire time.” she describes.

Dr. Garcia-Webb says most people will feel the effects within a few hours after taking phentermine. “I will start someone on a lower dose of phentermine (15mg daily) for a week to make sure they don’t have any significant side effects,” she says. “If they tolerate it well, I will increase to the maximum dose, 37.5mg daily, and then have them stay there.” When taking phentermine, Dr. Garcia-Webb recommends taking it in the morning because it’s a stimulant, and taking it later in the day can interfere with sleep. Patients with baseline insomnia should be aware of this stimulant effect of phentermine before starting this medication, as the sleep difficulty may worsen.

How long phentermine stays in your system varies from one person to another, based on their metabolism and any underlying medical conditions they may have, says Dr. Garcia-Webb. In particular, how phentermine is metabolized may be affected by kidney function or impairment. “Phentermine is primarily eliminated via the kidneys,” she explains. “If a patient has more acidic urine, it will be eliminated faster than if the urine is more alkaline.”

Does phentermine build up in your system?

Phentermine is an FDA-approved appetite suppressant. When appetite is reduced, fewer calories are consumed, and weight loss may result. “It’s unknown exactly how phentermine suppresses appetite, but it’s believed to work by raising neurotransmitter levels in the brain,” says Dubinsky. “The chemical messengers in your body are called neurotransmitters (norepinephrine, serotonin, and dopamine).” When your body produces more of these substances, you experience less hunger, Dubinsky explains.

You might wonder if phentermine builds up in your system after taking it. The answer is that it generally is not a medication that builds up over time in your body, says Dr. Garcia-Webb. She notes that Phentermine wears off much faster than an injectable glucagon-like peptide-1 receptor agonist (GLP-1), another popular weight loss medication.

GLP-1 agonists such as semaglutide (Ozempic and Wegovy) remain in the body for approximately five weeks after the last injection

Dr. Garcia-Webb notes that over time, some people taking phentermine can develop a tolerance to it so that the appetite suppression effect on them lessens. It’s also important to note that phentermine can cause side effects if you take it for several weeks. Dr. Garcia-Webb suggests watching for common side effects of phentermine. Potential side effects could include an increased heart rate, elevated blood pressure, dry mouth, anxiety, headaches, constipation, and trouble sleeping. Contact your healthcare provider if you have any phentermine side effects.

Phentermine detection methods

People taking phentermine often want to know if it will be detected in lab tests. Yes, phentermine can be detected in urine tests, mouth swabs, and hair tests.

“Daily drug tests, such as urine tests, swab samples from the mouth, hair tests, will detect phentermine and remain detectable in your system even after your phentermine intake,” says Kevin Huffman, DO, doctor of osteopathic medicine, bariatric healthcare provider, and CEO of On the Rocks Climbing Gym

According to Dr. Huffman, phentermine remains detectable:

In urine, about four days after the last dose
In oral swabs, about 48 hours after the last dose
In hair follicle samples, about 90 days from the last dose

Having phentermine in your body when labs are drawn is usually not an issue, especially if you were prescribed phentermine by a healthcare provider to manage your weight and health. The problem is if phentermine is detected in blood or urine, it may be confused for an amphetamine. “A person taking phentermine could test positive for amphetamines on a drug screen,” Dr. Garcia-Webb notes. However, this is what could be described as a “false-positive” result and doesn’t mean you have amphetamines in your system.

What happens when you stop taking phentermine?

Most healthcare professionals recommended only taking phentermine for several weeks and not on a long-term basis. So, what exactly happens when you stop taking phentermine? Will you immediately regain the weight you lost?

“Once a patient stops taking phentermine, the appetite suppressant effect wears off after a few days,” says Dr. Garcia-Webb. “A patient who discontinued phentermine will not regain the weight immediately, but it’s likely to creep back on over time.”

However, you aren’t powerless when it comes to gaining weight back after stopping phentermine. You may start to regain weight after you stop taking phentermine, but that’s usually if you return to unhealthy eating habits, says Dr. Huffman. “So be sure to continue your diet and exercise plan after taking phentermine to lose weight and keep it off,” he advises.

It’s important to understand that phentermine is a Schedule IV controlled substance with addictive qualities, additional administrative duties for the prescribing provider, and additional rules for pharmacies. As such, some individuals have trouble weaning themselves off phentermine. If you are experiencing this, please contact your healthcare provider for help and guidance, as it’s generally not advised to stay on phentermine for long periods of time.

You may be concerned that getting off of phentermine could cause withdrawal symptoms. “With short-term use, stopping phentermine normally doesn’t result in withdrawal symptoms,” says Dubinsky.

Dr. Garcia-Webb agrees that there usually aren’t significant withdrawal concerns. “This is different for patients who are taking the combination pill Qsymia, which is a blend of phentermine and topiramate, an anti-seizure medication, which does need to be tapered due to the effects of topiramate,” she notes.

If you do experience withdrawal symptoms after stopping phentermine, it’s usually after abruptly stopping a high dose of phentermine. In this case, your withdrawal symptoms might include:

Depression
Serious fatigue
Trouble sleeping
Irritability
Hyperactivity
Personality changes
Psychosis that might resemble schizophrenia

How to flush phentermine from your system before surgery

If you are going to have surgery that involves general anesthesia, it’s usually recommended that you discontinue the use of phentermine before surgery. This is due to a potentially harmful interaction between phentermine and general anesthesia you may receive, which could result in abnormally low blood pressure.

Since phentermine stays in your system for about four days after you stop taking it, it’s usually recommended that you stop taking it about a week before any surgery, says Dubinsky. Dr. Garcia-Webb agrees with that recommendation. “There can be some interactions with anesthesia, so patients will likely need to stop their phentermine 4-7 days before surgery, according to their surgeon,” she says.

Still, the timing of when to stop taking phentermine before surgery may depend on the surgery you are having and how much phentermine you are taking. “A patient undergoing surgery will have a conversation with their surgeon well before their surgery regarding which medications to stop and when to stop before surgery,” Dr. Garcia-Webb says.

Unfortunately, there isn’t a quick-fix way to remove phentermine from your body before surgery, such as a detox or other type of elimination regimen. “There is not a reliable way to effectively flush phentermine out of your body,” Dr. Garcia-Webb says. Instead, you just need to plan and stop taking phentermine about a week prior to surgery or within whatever timeframe your surgeon advises.