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Author: Admin | 2025-04-28
Published Sep 30, 2022 Last updated Feb 26, 2025 Key points Phentermine can be dosed more than once a day, depending on the dose your doctor prescribes. The FDA has approved phentermine to be dosed at either 15-37.5mg once daily or 18.75mg twice daily. AnswerAlthough 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 daily18.75mg twice dailyIt 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 DayPhentermine 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
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