This sheet talks about exposure to phentermine in a pregnancy and while breastfeeding. This information should not take the place of medical care and advice from your healthcare provider.

What is phentermine?

Phentermine is a medication that has been used as an appetite suppressant to treat obesity. Phentermine is sold under trade names such as Adipex®, Fastin®, Ionamin®, Lomaira™®, or Zantryl®. It is also an ingredient in Qsymia®.

Is phentermine the same as Fen-Phen?

No. Fen-Phen was a medication made of fenfluramine and phentermine prescribed to help people lose weight. Fenfluramine was taken off of the market in September 1997 because of the concern that it caused heart problems. Phentermine is still available.

Is it safe to lose weight while I’m pregnant?

It is not recommended that you lose weight during pregnancy. Some studies suggest that dieting during early pregnancy may increase the chance of having a baby with a neural tube defect (when the baby’s spine does not form properly). Spina bifida is an example of a neural tube defect. However, being significantly overweight increases the chance of some medical complications in pregnancy. These include gestational diabetes (diabetes that develops and lasts only during pregnancy) and high blood pressure. Your healthcare provider can discuss recommendations about weight gain or loss during pregnancy.

I just found out I am pregnant. Should I stop taking phentermine?

Talk with your healthcare providers before making any changes to how you take this medication.  Phentermine has not been associated with negative side effects when people stop taking it immediately. Therefore, a person could likely stop taking phentermine at any time. However, other appetite suppressants need to be slowly stopped over a period of several days to avoid side effects.

Can taking phentermine make it harder for me to get pregnant?

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

Does taking phentermine increase the chance of having a baby with a birth defect?

In every pregnancy, there is a 3-5% chance of having a baby with a birth defect. This is called her background risk. There is little information on phentermine exposures during early pregnancy. Information on women who used phentermine in combination with fenfluramine during the first trimester of pregnancy does not report an increase in the chance of having a baby with a birth defect.

Could taking phentermine in the second or third trimester cause other pregnancy complications?

Studies have not been done to see if phentermine increases the chance for pregnancy complications when used in the 2nd or 3rd trimester.

Does taking phentermine in pregnancy cause long-term problems in behavior or learning for the baby?

Studies have not been done to see if phentermine increases the chance for long-term problems.

Can I breastfeed while taking phentermine?

Phentermine use while breastfeeding has not been studied. We do not know how much phentermine gets into breast milk, or what effects it can have on a baby who is breastfed. Based on what is known about the medication in adults, possible side effects in the baby might tremors, agitation or decreased feeding. Talk with your healthcare provider if you are taking this medication and about all of your breastfeeding questions.

If a man takes phentermine, could it affect his fertility (ability to get partner pregnant) or increase the chance of birth defects?

There are no studies looking at possible risks to a pregnancy when the father uses phentermine. In general, exposures that fathers have are unlikely to increase risks to a pregnancy. For more information, please see the MotherToBaby fact sheet on Paternal Exposures at


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