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Is phentermine safe during pregnancy?

Answered by MotherToBaby’s teratology specialists, quoted in full — including the questions most pages skip, like whether a father’s exposure matters.

By the pharmaranks editorial teamReviewed against the MotherToBaby fact sheet for phentermine (OTIS, published via the NIH) sourcesUpdated Jul 21, 2026How we research

The questions people actually ask about phentermine in pregnancy

Answered by MotherToBaby, the fact sheets written by the Organization of Teratology Information Specialists and published through the NIH. Quoted in full — we do not condense them.

I am taking phentermine, but I would like to stop taking it before getting pregnant. How long does the drug stay in my body?

The time it takes the body to metabolize (to process) medication is not the same for everyone. In healthy non-pregnant adults, it takes up to 8 days, on average, for most of the phentermine to be gone from the body.

I take phentermine. Can it make it harder for me to get pregnant?

Studies have not been done to see if taking phentermine could make it harder to get pregnant.

Does taking phentermine increase the chance of miscarriage?

Miscarriage is common and can occur in any pregnancy for many different reasons. Studies have not been done to see if phentermine can increase the chance of miscarriage.

Does taking phentermine increase the chance of birth defects?

Birth defects can happen in any pregnancy for different reasons. Out of all babies born each year, about 3 out of 100 (3%) will have a birth defect. We look at published data to try to understand if an exposure, like phentermine, might increase the chance of birth defects in a pregnancy. There is little information on phentermine exposures during early pregnancy. The available data does not suggest an increased chance of birth defects.

Does taking phentermine in pregnancy increase the chance of other pregnancy-related problems?

Studies have not been done to see if phentermine increases the chance for pregnancy-related problems such as preterm delivery (birth before week 37) or low birth weight (weighing less than 5 pounds, 8 ounces [2500 grams] at birth).

Does taking phentermine in pregnancy affect future behavior or learning for the child?

Studies have not been done to see if phentermine can increase the chance of behavior or learning issues for the child.

If a male takes phentermine, could it affect fertility or increase the chance of birth defects?

Studies have not been done to see if phentermine could affect male fertility (ability to get partner pregnant) or increase the chance of birth defects. In general, exposures that fathers or sperm donors have are unlikely to increase the risks to a pregnancy. For more information, please see the MotherToBaby fact sheet Paternal Exposures at https://mothertobaby.org/fact-sheets/paternal-exposures-pregnancy/ .

Quoted from MotherToBaby Fact Sheets — Phentermine, published June 2024. MotherToBaby states its fact sheets should not take the place of medical care and advice from your healthcare providers.

This is general reference information, not medical advice. Whether to take any medication in pregnancy is a decision for you and your prescriber; do not start or stop a drug on your own, because an untreated condition can also carry risk.