How Long Does Adipex Stay in Your System – All You Need to Know

Those who are diagnosed with obesity might be prescribed Adipex to help them lose weight. It is administered in short intervals and could be addictive if taken for a long time. Adipex comes with side effects and may trigger a false positive on a drug test.

So, how long does Adipex stay in your system? The answer depends on numerous factors. Keep reading to find out more about how Adipex works, the half-life, possible side effects, and precautions.

What Is Adipex

Adipex is a brand name for phentermine, a weight loss medication. FDA approved phentermine back in 1959 for people aged 16 and older. It is to be used in treatment cycles of up to 12 weeks.

Phentermine is a controlled drug and can only be obtained with a prescription in the United States. It shares certain chemical properties with amphetamine, a stimulant.

Phentermine, including Adipex, is prescribed to obese people who desire to lose weight. According to a study published in 2015, a patient should have a body mass index of over 30 for a prescription to Adipex. The same study posits that in some cases, especially in the presence of other obesity-related health issues, patients with body BMI of 27 can also be prescribed Adipex. The side issues include but are not limited to high cholesterol, hypertension, and type 2 diabetes.

How Does It Work?

Phentermine is an anorectic, a drug that’s used to suppress a patient’s appetite. It lowers your feeling of hunger and thus limits the number of calories you take. The lower calorie intake would lead to weight loss.

Though the mechanism of Adipex remains unclear, it is suspected that it boosts the levels of neurotransmitters in your brain. When the levels of dopamine, serotonin, and norepinephrine rise to a certain level, they mask and suppress your feeling of hunger, which should promote weight loss.

The downside to Adipex is that patients may develop a tolerance to the drug over time. It usually takes a few weeks before the effects of Adipex would start diminishing. However, it is not recommended for patients to increase the dosage on their own.


Adipex has a stimulating effect and is known to raise hormone levels. Therefore, users run a risk of becoming addicted. That’s partially why the maximum treatment period is limited to 12 weeks. It should also be noted that people who have a history of drug abuse may not be eligible for Adipex or phentermine.

Drugs that might interact with Adipex include antidepressants and related drugs. As is the case with all drugs, you should let your doctor know if you take monoamine inhibitors or selective serotonin uptake inhibitors.

Also, if you are taking insulin-based medications and guanethidine for depression and weight loss, be sure to disclose the fact. Stop taking Adipex if you suspect that you’re allergic to phentermine.

Over-65 seniors, pregnant women, women who breastfeed, and women who plan on becoming pregnant shouldn’t take phentermine. Finally, it is recommended not to drink alcohol while under the effects of Adipex.

Half-Life and Elimination

How long does Adipex stay in your system? The half-life for Adipex is approximately 25 hours. It is a primarily renal excretion drug, i.e. it leaves the body through urine. A study from 2013 found that around 4/5th of the drug is eliminated in unchanged form. The half-life of the drug is longer in patients that have alkaline urine.

The time needed to eliminate the drug from the system varies from patient to patient. On average, it is around 5.5 half-lives. You should have in mind that there are various factors at play here. How long it takes to leave your body depends on your build, age, health, dosage, and metabolic rate.

Adipex can cause you to test positive on a urine drug test due to its chemical resemblance to amphetamine. If that happens, you can request to take a confirmatory test to determine that it was phentermine (Adipex) in your system and not amphetamine. In essence, you’ll be proving that it was a false positive.

Adipex can be detected in blood tests for 24 hours after the last dose and up to four days in urine tests. It can also show up in hair tests for up to 30 days.

If you expect to take a drug test while on Adipex, you should report that to make sure that the lab interprets your results properly.


Up until 2016, Adipex and other phentermine brands were available in 37.5, 30, and 15mg tablets. In 2016, the FDA standardized it to 8mg. This new formulation can be administered up to a maximum of three times a day.

However, if you’re taking three doses a day, you may not want to take the last one late in the evening for the risk of sleeping problems, such as an inability to fall asleep and difficulties staying asleep.

The FDA recommends the minimum effective therapy. If your doctor prescribes Adipex in combination with topiramate, you will be on the lowest dose for two weeks. After that, the doctor might opt to increase the dose based on how effective the drug was in the past 14 days.

If you fail to lose even 5% of your weight after taking the maximum recommended dose for 12 weeks, you might as well discontinue the treatment.

Side Effects

Adipex can only be used for short periods of time when administered alone. Therefore, there’s a lack of data regarding the long-term effects and risks. Mild side effects might include unpleasant taste and dry mouth. Vomiting, constipation, and diarrhea are also not uncommon.

It should be noted that you’re not supposed to open the capsule, cut it, or crush it. This would cause an immediate release and you might experience some of these side effects:

  • Tremors
  • Chest pain
  • Swollen ankles and legs
  • Restlessness
  • Heart palpitations
  • Shortness of breath
  • Trouble sleeping
  • High blood pressure

For cases of severe overdose, according to the FDA, side effects might include assaultiveness, rapid breathing, arrhythmia, circulatory collapse, abdominal cramps, and more. Hyperflexion, hallucinations, and social dysfunction may also happen.

In Combination with Topiramate

Adipex is sometimes combined with topiramate for long-term usage. When administered in parallel, both drugs are taken in lower doses than when administered individually. Though severe side effects rarely happen, studies have found a number of milder side effects. These include:

  • Sleeping problems
  • Heart palpitations
  • Constipation
  • Fatigue
  • Irritability
  • Skin flushes
  • Dizziness
  • Dry mouth

Final Words

Adipex can be very helpful but it can also cause a slew of side effects and health problems, especially if it interacts with other therapy drugs. It has a half-life of about 25 hours and takes about 5.5 half-lives to completely exit the body. This would depend on the patient including health status, age, and dosage. Drug tests have been known to register traces of Adipex in the patient’s hair for up to one month after the last use.