Can You Take Phentermine Twice A Day?
In our latest question and answer, the pharmacist discusses taking phentermine two times a day.
I work nights. If I take Phentermine 37.5 mg on Thursday night, can I take 18.75 on Friday morning when I am off? Then I would take the other 18.75 mg late afternoon (approximately 8 hours later).
Sep 30, 2022- Phentermine can be taken twice daily at a dose of 18.75mg, though once daily dosing is more common.
- Taking phentermine later in the day, especially as a second dose, often causes insomnia because of how long the drug stays in your body.
- Talk to your doctor about the best dosing schedule for your individual situation.
Answer
Although phentermine is much more commonly dosed once daily, it certainly can be taken twice daily, depending on the dose you are prescribed.
The drug is approved by the FDA for the following dosing:
- 15mg to 37.5mg once daily
- 18.75mg twice daily
It is important to note that phentermine has a long duration of action, around 12 to 14 hours depending on the individual. While sometimes taking a lower dose (e.g. 18.75 milligrams) can result in a shorter duration of action, this isn't always the case. This is mostly due to how our bodies metabolize drugs.
For most drugs, metabolism and elimination rate are not closely correlated to the dose taken. For example, the half-life (the time it takes for 50% of a drug to be metabolized) of phentermine is around 24 hours.
It generally takes around 5 half-lives for a drug to be considered completely eliminated from the body. This holds true, regardless of the dose taken. Therefore, 37.5mg of phentermine will take about as long as 18.75mg of phentermine to be completely metabolized, around 5 days.
Having said this, many individuals report that taking two doses of phentermine, one in the morning and one in the early afternoon, better helps reduce appetite versus a once-daily dose.
Caution Taking Phentermine Later In The Day
Phentermine is generally recommended to be taken once a day, in the morning before breakfast. Taking it later in the day often causes insomnia, thanks to its long duration of action.
However, if tolerated, splitting the phentermine 37.5mg tablet in half (and taking 18.75mg in the morning and 18.75mg early in the afternoon) may be an option for you and it is indicated to be dosed this way, for certain individuals. The main area of concern is insomnia it may cause, especially when dosed later in the day. If splitting your tablet, most phentermine tablets are scored, to ease cutting or splitting them in half.
It is important to talk to your doctor regarding your dosing of phentermine as they know your medical situation more completely.
Phentermine Information
Phentermine is used for a limited period of time to speed weight loss in overweight people who are exercising and eating a low-calorie diet. It is in a class of medications called anorectics. It works by decreasing appetite.
Final Words
Thanks for reaching out to us! I hope this helped.
References & sources
Glossary — Terms in This Answer
- FDA
- The Federal Drug Administration, the government agency that approves medications for use in the United States.
- Duration of action
- The length of time a drug remains active and effective in your body.
- Metabolize
- The process by which your body breaks down a drug into substances it can eliminate.
- Half-life
- The time it takes for the amount of a drug in your body to reduce to half of its original level.
- Insomnia
- Difficulty falling asleep or staying asleep.
- Scored
- Having a line marked on a tablet to make it easier to split into smaller pieces.
- Anorectics
- A class of medications that reduce appetite.
- Low-calorie diet
- A diet plan that restricts the total amount of calories consumed each day.
Dr. Brian Staiger, PharmD
Doctor of Pharmacy, board certified clinical pharmacist (BCPS), American Herbalist Guild, Medication Therapy Management certified
Brian Staiger, PharmD, BCPS, is a licensed pharmacist with more than 13 years of experience across retail pharmacy, clinical pharmacy, medication safety, pharmacy program development, and healthcare administration. He is the founder of HelloPharmacist.com, where he helps patients...
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