Phentermine dosage, forms, and strengths

Written by Chad Shaffer, MD
Physician
Updated Feb 5, 2025  •  Published Jun 5, 2023
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Phentermine’s past is marked by its prior inclusion in the combination weight loss drug fen-phen, which was infamously removed from the market by the Food and Drug Administration (FDA) in the 1990s due to its association with valvular heart disease. Those adverse reactions were attributed to fenfluramine, the other drug in fen-phen, and present-day phentermine remains on the market and is commonly used for short-term weight management. 

Although heart valve side effects were not pinned on phentermine, it does have the potential for other serious side effects and heart problems, including hypertension (high blood pressure), tachycardia (elevated heart rate), heart palpitations, chest pain, dependency, and abuse. For that reason, proper dosing is of paramount importance. Dosage forms of Phentermine include 15 mg, 30 mg, 37.5 mg capsules, and 37.5 mg tablets. A brand-name formulation, Lomaira, is available as an 8 mg tablet. Common side effects of phentermine include mood or behavior change, restlessness, fast heart rate, constipation, dry mouth, and unpleasant taste.

RELATED: Phentermine side effects and how to avoid them

Phentermine forms and strengths

  • Capsule (generic): 15 mg, 30 mg, 37.5 mg

  • Tablet (generic or Adipex-P): 37.5 mg

  • Tablet (Lomaira): 8 mg

Phentermine dosage for adults

Phentermine is a Schedule IV drug approved by the FDA for short-term weight management treatment, with the treatment duration generally regarded as 12 weeks. On the contrary, Qsymia is a prescription that combines phentermine and topiramate and is approved for chronic weight management. 

According to the drug’s label, candidates for the drug include adults with a body mass index (BMI) of at least 30 kg/m^2, classifying them as obese, or those with a BMI of at least 27 kg/m^2 coupled with weight-related comorbidity. Comorbidities attributable to obesity or being overweight can range from high cholesterol and diabetes mellitus to knee osteoarthritis and obstructive sleep apnea.

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Phentermine dosage chart
Indication Starting dosage Standard dosage Maximum dosage
Short-term weight management treatment 15 mg once daily 15-37.5 mg once daily 37.5 mg once daily
 

RELATED: Is phentermine for weight loss safe?

Phentermine dosage for weight loss

To assist with weight loss, phentermine can be started at any of the dose strengths or the lowest dose, 15 mg once daily, and then increased to the maximum dosage of 37.5 mg once daily. The daily dose can also be divided into two dosings, such as 15 mg twice daily, as long as it does not exceed the maximum amount of 37.5 mg. These dosages can achieve an appetite suppressant effect by being a central nervous system stimulant (CNS stimulant). Phentermine acts like an amphetamine and induces the brain's hypothalamus region to release more norepinephrine.

Phentermine dosage for children

Phentermine is only approved for children over age 16 with a BMI of at least 30 kg/m^2 or those with a BMI of at least 27 kg/m^2 coupled with weight-related comorbidity. Like with adults, the only FDA indication is short-term weight management treatment.

  • Standard phentermine dosage for children older than 16: 15-37.5 mg once daily

  • Maximum phentermine dosage for children older than 16: 37.5 mg once daily

RELATED: AAP releases new clinical guidelines for childhood obesity

Phentermine dosage restrictions

It is important to recognize when phentermine should not be used. It is contraindicated in a number of medical history settings, including cardiovascular disease, uncontrolled hypertension, pulmonary hypertension, hyperthyroidism (overactive thyroid), a history of drug abuse, glaucoma, agitation, pregnancy, breastfeeding, and advanced atherosclerosis. In addition, caution is advised about using it in the setting of controlled hypertension, coronary artery disease, diabetes, and older adults. While dosing adjustments are not defined for hepatic (liver) disease, recommendations for those with renal impairment (kidney disease) and other risk factors are available.

  • Renally impaired with creatinine clearance 15-29 mL/min: 15 mg once daily maximum dose

  • Renally impaired with creatinine clearance <15 mL/min: Avoid use

How to take phentermine

The timing of phentermine dosing is best 30 minutes before breakfast or one to two hours after. If being dosed twice daily, maintaining this timing both before breakfast and supper would work. Late evening dosing should be avoided. Between dosings, pills should be securely stored to avoid accidental ingestion by young children. This is also worthwhile because phentermine is listed as a controlled substance by the Drug Enforcement Agency (DEA) based on its potential for abuse. Some of the details of phentermine use are listed below:

  • A healthcare provider will inform patients how much medicine to use. Do not use more than directed.

  • This medicine is not for long-term use.

  • To avoid trouble sleeping, always take this medicine in the morning and never at bedtime or late in the evening. Take the phentermine tablet before breakfast or 1 to 2 hours after breakfast. Take the Lomaira tablet 30 minutes before meals.

  • Tablet: Swallow whole. Do not crush, break, or chew it.

  • Missed dose: Take a dose as soon as remembered. If it is almost time for the next dose, wait until then and take a regular dose. Do not take extra medicine to make up for a missed dose.

  • Carefully follow all of the doctor's instructions and medical advice about any special reduced-calorie diet.

  • Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light.

  • This medicine is not for long-term use.

Phentermine dosage FAQs

Phentermine and Adipex-P vs. Lomaira: What’s the difference in doses?

Adipex-P is simply a brand-name version of phentermine and is available in the typical doses listed above. Lomaira, on the other hand, is a low-dose, brand-name version meant to be taken as an 8 mg tablet three times daily with meals. Brand-name options can be more expensive. The best way to save money is with SingleCare coupons or a SingleCare discount card.

How long does it take phentermine to work?

Phentermine works quickly to reduce appetite. Peak concentrations of the drug are typically reached within four hours. Taking the drug on an empty stomach is best; taking it 2 hours after breakfast is common.

How long does phentermine stay in your system?

Based on a half-life of 24 hours, half of the bodily concentration of phentermine will be eliminated within a day. By five days from the last dose, 97% of the drug is eliminated.

What happens if I miss a dose of phentermine?

If a missed dose of phentermine is remembered shortly after its scheduled time, it can be taken, but if it is already late in the day, it is best to wait until the following day. Never double up on a dose of phentermine.

How long can you take phentermine?

The maximum duration of phentermine use is usually considered to be 12 weeks. This timeframe is considered concordant with the FDA indication that the drug is limited to short-term weight management. It also matches clinical trial evidence showing that phentermine produced 17 pounds of average weight loss in obese patients compared to 4 pounds with placebo in a 12-week study.

How do I stop taking phentermine?

Stopping phentermine suddenly has been a concern due to withdrawal symptoms like extreme fatigue and depression experienced with amphetamines. A withdrawal syndrome was not identified in clinical trials. Nonetheless, it is best to ask the doctor about how to discontinue the drug safely. 

What is the maximum dosage for phentermine?

The maximum daily dose of phentermine is 37.5 mg. Whether it is dosed once daily or divided into multiple doses per day, the total daily dose should not exceed 37.5 mg.

Can you overdose on phentermine?

An overdose of phentermine can occur and lead to serious or fatal effects. Some potential consequences are agitation, restlessness, vomiting, diarrhea, irregular heartbeat, and seizures. Getting emergency care and advice from the Poison Help Line at 1-800-222-1222 is in order.

What interacts with phentermine?

Phentermine has the potential to interact with a broad array of other medications, both prescription drugs and over-the-counter varieties. The seriousness of these interactions varies. Monoamine oxidase inhibitor (MAO inhibitor or MAOI) use within 14 days of phentermine is absolutely contraindicated as it may contribute to a hypertensive crisis. This restriction includes isocarboxazid, linezolid, phenelzine, procarbazine, selegiline, tranylcypromine, and yohimbe. Other drugs, including herbal products and dietary supplements, may need to be avoided or used cautiously.

What happens when you mix phentermine and alcohol?

The side effects of phentermine can be exacerbated by alcohol. The combination should be avoided.

Is it safe to take phentermine during pregnancy?

The use of phentermine is contraindicated in pregnancy, and nursing mothers should not use phentermine while breastfeeding. While data on use in these circumstances is limited, the potential for fetal and newborn harm is considered too high to justify use.

Written by Chad Shaffer, MD
Physician

Chad Shaffer, MD, earned his medical doctorate from Penn State University and completed a combined Internal Medicine and Pediatrics residency at the University of Pittsburgh Medical Center and Children’s Hospital of Pittsburgh. He is board certified by the American Board of Internal Medicine and the American Board of Pediatrics. He has provided full-service primary care to all ages for over 15 years, building a practice from start up to over 3,000 patients. His passion is educating patients on their health and treatment, so they can make well-informed decisions.

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