Q: I am trying to conceive (TTC) and taking phentermine. Could this be a problem?

A: Phentermine is a prescription medication used to treat obesity and increase weight loss. The U.S. Food and Drug Administration (FDA) uses a pregnancy category system to classify the possible risks to a fetus when a specific medicine is taken during pregnancy and phentermine was given a category C.

Pregnancy Category C is given to medicines that have not been studied in pregnant humans but do appear to cause harm to the fetus in animal studies.

Medicines that have not been studied in any pregnant women or animals are automatically given a pregnancy Category C rating.

Phentermine has not been studied in any pregnant women or animals. There have been a few reports of lung problems and stillbirths possibly related to phentermine use during pregnancy. No increase in birth defects was found in women who took Fen-Phen (a medication made of phentermine and fenfluramine, which was taken off the market because of concerns that it caused heart problems).

Phentermine is chemically related to amphetamines, a group of drugs that have been shown to increase the risk of birth defects and miscarriages. Since it is not known if phentermine is safe during pregnancy (and since taking phentermine is not absolutely necessary), it is not recommended for pregnant women in general or for women trying to conceive.

Some studies suggest an increased risk to the fetus if you lose weight during pregnancy, so in general, it’s not recommended to lose weight if you are pregnant. If you took phentermine during pregnancy, you should talk to your doctor about what tests can be done to determine if the fetus has physical birth defects.