Many Phentermine Capsules


There is no drug interaction between phentermine and Synthroid (levothyroxine). However, Synthroid is generally recommended to be taken by itself, on an empty stomach, 30 to 60 minutes before food or other medication for optimal absorption.

The prescribing information for Synthroid gives the following dosing recommendation:

  • Administer once daily, preferably on an empty stomach, one-half to one hour before breakfast.
  • Administer at least 4 hours before or after drugs that are known to interfere with absorption (e.g. calcium salts, antacids).

Taking Synthroid at the same time as phentermine could potentially decrease absorption and the overall effectiveness of the medication.

As Synthroid is a thyroid replacement medication, decreased absorption could precipitate hypothyroidism, which could bring on symptoms of fatigue, shivering etc...

So overall, you can take Synthroid and phentermine the same day, just be sure to separate dosing by an appropriate amount of time.

Answer Summary

Phentermine and Synthroid do not interact with one another. However, Synthroid is generally recommended to be taken on an empty stomach, 30 to 60 minutes prior to food or other medication.

Phentermine & Thyroid

Thyroid On Card

Since we are talking about phentermine and Synthroid, I wanted to add that phentermine does have some potential drug-disease interactions that are important to know about.

For starters, phentermine is contraindicated in those with hyperthyroidism.

Phentermine is known as a 'sympathomimetic' drug and is structurally related to amphetamines like Adderall. Using a sympathomimetic drug in those with high thyroid hormone levels increases the risk of serious adverse events, such as primary pulmonary hypertension and mitral valve dysfunction.

Additionally, although not contraindicated, caution is recommended in those with hypothyroidism as well. Phentermine can mask symptoms of hypothyroidism, potentially delaying proper treatment.

Some of the most common complaints of hypothyroidism include fatigue, feeling chilled, being lethargic and gaining weight. Phentermine, due to its amphetamine-like effects, can negate or improve these side effects, making it difficult to discern whether your hypothyroidism is being properly treated.

If you do you have hypothyroidism (and presumably you do if you are taking Synthroid), just be sure to always go for lab work when recommended by your doctor so your thyroid levels can be properly evaluated.

About Phentermine

Phentermine is classified as a sympathomimetic stimulant medication, chemically similar to amphetamines. It is most often used for the treatment of obesity and is one of the most commonly prescribed medications for weight loss.

While weight loss on phentermine is generally a long process, the amphetamine-like effects of phentermine are quickly noticeable after dosing, within 30 minutes.

Each dose will last from 4 to 14 hours, depending on the individual and the dose (37.5 mg has the longest duration of action).

Phentermine is recommended to be dosed in the morning to prevent issues of insomnia.

  1. Synthroid Prescribing Information. RxAbbvie
  2. Phentermine Prescribing Information. AccessFDA