In every pregnancy, a woman starts out with a 3-5% chance of having a baby with a birth defect. This is called her background risk. This sheet talks about whether exposure to phentermine may increase the risk for birth defects over that background risk. This information should not take the place of medical care and advice from your health care provider.

What is phentermine?

Phentermine is a medication used as an appetite suppressant to treat obesity. Women are sometimes prescribed medications such as phentermine to help them to lose weight. Phentermine is sold under trade names such as Adipex®, Fastin®, Ionamin®, and 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?

In general, it is not recommended that you lose weight during pregnancy. Some medical complications are more likely in pregnancy if you are significantly overweight, such as gestational diabetes (diabetes that develops and lasts only during pregnancy) and high blood pressure. Your healthcare provider can discuss recommendations regarding weight gain and loss during pregnancy. Some studies suggest that dieting during early pregnancy, without taking vitamins, may increase the chance of having a baby with a neural tube defect. Neural tube defects, such as spina bifida, occur when the baby’s spine does not form properly.

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

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

I stopped taking phentermine about one month before I became pregnant. Was my baby exposed to this medication?

The phentermine would have been gone from your body before you became pregnant; therefore your baby was not actually exposed to the phentermine that you took. Most of the phentermine should be gone from a person’s body four to five days after taking the last dose.

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

There is little information on phentermine exposures during early pregnancy. There is some information that looks at women who used phentermine in combination with Fen-Phen during the first trimester of pregnancy. This information does not report an increase in the chance of having a baby with a birth defect.

I used phentermine before I knew I was pregnant and am concerned about my baby. Are there any tests I could have to make sure my baby is ok?

An ultrasound examination can often be used to find out if the baby has physical birth defects. An ultrasound allows a health care provider to see many things, including the baby’s head and spine. However, ultrasound is not perfect and might not pick up all birth defects. You may also want to talk with your health care provider about the MSAFP test (maternal serum alpha fetoprotein test), a blood test that can be done in the beginning of your 2nd trimester and is used to screen for neural tube defects.

Now that I know I’m pregnant, can I stop taking the phentermine immediately?

Phentermine has not been associated with negative side effects when people stop taking it immediately. Therefore, a person could stop taking phentermine at any time. Other appetite suppressants need to be slowly stopped over a period of several days to avoid side effects. However, you should always talk with your healthcare provider before stopping any prescription medications.

I’ve heard Fen-Phen causes heart problems in adults. Will phentermine do the same to my baby?

Fenfluramine, as part of Fen-Phen, is thought to cause heart problems in adults. There are no specific studies that look to see if phentermine affects the developing baby’s heart. The limited available information suggests it would be unlikely for phentermine to cause heart problems in babies exposed during pregnancy.

I gained a lot of weight while I was pregnant. I am thinking of using phentermine to lose the weight. Can I still breastfeed my baby?

It is possible for phentermine to pass into breast milk, so a baby could be exposed while nursing. 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. Because phentermine stimulates the brain, it might cause side effects in the baby such as tremors, agitation or decreased feeding. Talk with your healthcare provider if you are taking this medication. Be sure to talk with your healthcare provider about all of your breastfeeding questions.

What if the father of the baby uses phentermine?

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


  • Aronne LJ. 1998. Obesity. Womens Health Issues 82(1):161- 181.
  • Hale TW. 2008. Medications and Mothers’ Milk, 13th ed. Amarillo, TX: Hale Publishing.
  • Johnson KA, et al. 1998. Pregnancy outcome of women exposed to Fen/Phen [abstract]. Organization of Teratology Information Specialists Annual Meeting, June 1998.
  • Jones KL, et al. 2002. Pregnancy outcomes after first trimester exposure to phentermine/fenfluramine. Teratology 65:125-130.
  • No authors listed. 2013. Topiramate + phentermine. An excessively dangerous appetite-suppressant combination. Prescrire Int 22(136):61-4.
  • Robert E, et al. 1995. Neural tube defects and maternal weight reduction in early pregnancy. Reprod Toxicol 9(1):57-59.
  • Silverstone, T. 1982. Drugs and Appetite. New York: Academic Press.
  • Stein AD, et al. 1995. Third-trimester pregnancy weight gain and intrauterine growth: The Dutch Famine Birth Cohort Study. Hum Biol 67(1):135-150.
  • Winick M. 1989. Nutrition, Pregnancy, and Early Infancy. Baltimore: Williams & Wilkins.