Phentermine, a weight loss drug, is not safe to take during pregnancy. If a person unexpectedly gets pregnant while taking the medication, they should stop immediately to prevent complications.

Pregnant people should not take phentermine (Adipex-p) because deliberate weight loss is not usually safe during pregnancy.

Researchers do not know all of the potential side effects of this drug, but the small amount of available data suggests it may harm a growing fetus. If a person does get pregnant while taking phentermine, they should contact a doctor.

Read on to learn more about phentermine and pregnancy.

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A person should not try to become pregnant while taking phentermine.

This is because most birth defects develop early in pregnancy, and often, appear before a person even learns they are pregnant.

People who use phentermine for weight loss should consider using contraception to prevent unintended pregnancy. If they do become pregnant, they should discontinue using phentermine and consult a doctor immediately.

Phentermine is a prescription medication that works by suppressing the appetite. While doctors generally consider it safe for most people, they do not recommend it for people who are pregnant, may become pregnant, or are nursing.

A 2021 review reports that 4.9% of a large sample of 30,704 pregnant people had an exposure to anti-obesity drugs during pregnancy. Phentermine was the most common drug, accounting for 15.5% of exposures.

The prescribing insert advises against using phentermine during pregnancy. It urges pregnant people to discontinue use of the drug.

There are two main reasons why phentermine is unsafe during pregnancy. First, doctors do not know all of the potential complications. Second, weight loss during pregnancy is usually unsafe for both the parent and the fetus.

The limited body of available evidence suggests that phentermine may increase the risk of complications, such as cleft palate and other birth defects.

While experts do not know all of the potential risks of taking phentermine during pregnancy, they are aware of some. It is unethical to conduct placebo-controlled trials of human use of phentermine during pregnancy, so quality research cannot directly test the risks.

Risks to the fetus

It may increase the chances of an infant being larger than average for its gestational age. A 2021 review found that more infants were at least 4,000 grams (8.8 pounds) larger for their gestational age when their parent used any anti-obesity drug during pregnancy. The review also found that infants may have a higher birth weight when exposed to anti-obesity drugs during development.

High birth weight increases the risk of some complications, such as shoulder dystocia, which occurs when a baby gets stuck during birth.

The same review also found a correlation between phentermine use and thickened mitral valves of the heart and a type of brain cyst, bilateral porencephalic cysts. While the study did not prove that phentermine caused these complications, the drug may increase the risk.

A 2019 case report also suggests a correlation between phentermine use and stroke in a developing fetus.

Risks to the parent

There may also be risks to the pregnant parent.

Deliberate weight loss during pregnancy is not safe, even in people who are overweight or have obesity. Instead, doctors generally advise people to minimize weight gain, remain active, and eat a balanced diet.

Phentermine may increase the risk of gestational diabetes, which poses risks to both the developing fetus and the pregnant person.

Researchers do not know if phentermine is safe during nursing or how much of the drug might get into milk. This is partially because placebo-controlled trials to test phentermine in postpartum individuals would be unethical and potentially unsafe.

Because phentermine is a stimulant, and stimulants are potentially harmful to infants and young children, people should generally not take it while nursing. For most people, the risks outweigh the benefits since phentermine is not a drug that is vital to someone’s health.

People using, or thinking of using, phentermine, who are considering becoming pregnant should ask a doctor:

  • How long should I be off of phentermine before trying to become pregnant?
  • What are my healthiest options for weight management during pregnancy?
  • If I have exposure to phentermine during pregnancy, what should I do to reduce the risk of birth defects?
  • What is my target weight during pregnancy?
  • Are there alternatives to phentermine in the preconception period?
  • Are there any side effects associated with suddenly stopping phentermine? What should I do to manage them?

Phentermine is not safe during pregnancy.

Because birth defects can develop before a person knows they are pregnant, it is not safe for people trying to become pregnant to take phentermine.

People trying to lose weight should discuss their options with a doctor before becoming pregnant. After a person becomes pregnant, deliberate weight loss can increase the risk of a range of complications.

If a person has taken phentermine during pregnancy, even unintentionally, they should tell all medical professionals.