Phentermine Dosage & Administration

Phentermine Dosage & Administration

People on the weight control medication phentermine are often looking for information about the medication. There is a significant amount to know and learn about this drug before taking it. Phentermine dosage and administration are some of the topics of interest, as well as a few others. Let’s take a look at some of the more relevant aspects.

Dosage & Indications

Phentermine is meant for short term (few weeks) treatments of exogenous obesity. For adults and adolescents at least 17 years of age, it is consumed orally, typically in 15 to 37.5 mg doses once per day. It is recommended that it is taken as a single dose every morning after breakfast (ideally around 2 hours after). 1

Some of the tablets with higher dosages can be divided, with 18.75 mg (half tablet) given in the morning, and the other later in the day (but not in the late evening hours) if it is necessary. In fact, for some patients, 18.75 mg per day will be sufficient. Geriatric patients taking phentermine may require a lower dosage. 2

Dosages can also be in 8mg tablets taken 3 times per day, prior to meals. Geriatric patients should be started at lower doses (perhaps 4mg 3 times a day), as they may require lower doses as a cautionary usage recommendation. Therefore the lowest effective dose is the typical standard and late evening consumption is not recommended to avoid cases of insomnia.

Phentermine is intended for use by patients with a BMI of 30kg/m2 or more, as well as those with a BMI of 27kg/m2 or more, as long as there is also a presence of at least 1 other weight-related health risk factor. This can include diabetes mellitus, controlled hypertension, or dyslipidemia. 3

According to the American Association of Clinical Endocrinologists and American College of Endocrinology (AACE/ACE) Obesity Clinical Practice Guidelines, there are not enough benefits from phentermine to justify a longer-term treatment in obese and overweight patients, so a short term therapy with the medication is typically recommended. 4

Maximum Dosage

The maximum recommended dosage breaks down by age range and should be provided depending on the person’s unique situation. 5

  • Adults: 37.5 mg/day PO
  • Elderly: 37.5mg/day PO, but should be considered for a lower dose than younger adults
  • Adolescents: Over 16 years old, 37.5mg/day PO. 16 years or younger, efficacy and safety have not yet been established.
  • Children: Efficacy and safety have not yet been established.

Dosing Considerations

The effects of hepatic impairment (liver failure) due to the use of phentermine are unknown. When it comes to renal impairment (kidney failure), an 8 mg dose of phentermine is considerable. The drug needs to be taken with caution. 6

While there are no specific guidelines available for dosage adjustments for patients with renal impairments, phentermine is reported to be excreted in the urine, so exposure increases can be expected. 7

When it comes to intermittent hemodialysis, some manufacturers recommend avoiding phentermine in patients with end-stage renal disease which requires dialysis (Adipex-P). Lomaira has no recommendations for dialysis patients.

Phentermine 15 mg and 30 mg tablets (generic manufacturers):

  • eGFR greater than 29 mL/min/1.73 m2: Requires no dosing adjustments.
  • eGFR 15 to 29 mL/min/1.73 m2: Should not exceed 15 mg/day PO.
  • eGFR less than 15 mL/min/1.73 m2: Avoid using.

Phentermine HCl 37.5 mg tablets and capsules equivalent to 30 mg of phentermine base (e.g., Adipex-P):

  • eGFR greater than 29 mL/min/1.73 m2: Requires no dosing adjustment.
  • eGFR 15 to 29 mL/min/1.73 m2: Should not exceed 15 mg/day PO.
  • eGFR less than 15 mL/min/1.73 m2: Avoid using.


Phentermine hydrochloride is administered orally in either Apidex-P or equivalent at 15mg to 37.5 mg tablets or Lomaira 8mg tablets.

The usual administration dose for 15 mg – 37.5 mg tablets are once daily, ideally 1-2 hours after breakfast, and approximately 30 minutes before meals for 8 mg doses, three times daily.

15 mg – 37.5 mg dosed tablets may contain a score which permits the daily dose to be divided in half, and then administered either before meals or 1 to 2 hours after meals twice daily. 8 mg dosed tables are scored for administration by half of the normal dosage for those patients who do not require a full dose.

None of the tables are recommended to be administered in the late evening to avoid insomnia.


Generic brands of phentermine, along with Adipex-P, Atti-Plex P, and Fastin should be stored between 68 and 77 degrees F, with the excursion levels permitted to range between 59 and 86 degrees F.

Atti-Plex P Spansule, Suprenza, and Lomaira should be stored at controlled room temperature between 68 and 77 degrees F.

Ionamin should be stored at 77 degrees F, allowing for a range of 59 -86 degrees F.

Pro-Fast should be stored at a controlled room temperature of 68 to 77 degrees F. It should be stored in its original container and protected from light. If the product contains particulate matters, is discolored or cloudy, it should be discarded.

Related Pages:


  1. Phentermine/Topiramate (Qsymia) for Chronic Weight Management
  2. What weight loss treatment options do geriatric patients with overweight and obesity want to consider?
  3. NIH: Prescription Medications to Treat Overweight and Obesity
  4. Comprehensive Clinical Practice Guidelines for Medical Care of Patients with Obesity (2016)
  5. Suprenza Highlights of Prescribing information
  6. Obesity Management in Adults with Chronic Kidney Disease
  7. The metabolism and urinary excretion in man of phentermine, and the influence of N‐methyl and p‐chloro‐substitution

Last updated: April 6th, 2020 by Sarah Gonzales. Bookmark the permalink.

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