Taking Phentermine While Pregnant: Risks You Should Know

Phentermine is one of the most commonly prescribed weight loss medications in the United States. Marketed under brand names like Adipex-P and Lomaira, it’s known for its appetite suppressant effects and ability to help certain healthy, non-pregnant adults lose weight quickly. But when it comes to taking phentermine while pregnant, the conversation shifts significantly toward risk, caution, and the need for close medical guidance.

Many pregnant women and those planning a pregnancy wonder whether continuing or starting weight loss drugs like phentermine could harm the developing fetus. The available published literature is limited, but what does exist suggests some crucial concerns.

Key Takeaways

• Taking phentermine while pregnant has been linked to an increased risk of birth defects and other pregnancy complications.
• Healthcare providers typically recommend stopping the medication altogether if you are pregnant or trying to conceive.
• There is only a limited amount of published data available on phentermine exposures during early pregnancy, making medical oversight critical.

Understanding Phentermine and Its Common Uses

Phentermine is primarily used for weight loss and appetite suppression for compatible individuals. It works by stimulating the central nervous system to reduce hunger, and it’s often prescribed for short-term use alongside diet and exercise. Phentermine is also available in combination medications like phentermine and topiramate, sold under the brand name Qsymia.

Approved for healthy non-pregnant adults struggling with obesity or weight-related medical conditions, phentermine increases norepinephrine levels in the brain, which suppresses appetite and boosts energy expenditure.

However, while this drug can be effective in managing weight loss, its use during pregnancy poses possible risks that far outweigh its benefits.

“Phentermine is not approved for use during pregnancy due to the potential for fetal harm. There is no clinical benefit to continuing this medication once a patient is known to be pregnant,” says Dr. Matthew Olesiak, MD, Chief Medical Director at SANE MD.

Health Risks of Taking Phentermine During Pregnancy

Many women may unknowingly continue taking phentermine into early pregnancy, especially if the pregnancy is unplanned. In some cases, the medication may have been prescribed before the woman was aware she was pregnant. Others may intentionally continue its use, believing it will help manage gestational weight gain. But both situations carry significant risk.

For a deeper look at how this medication can affect women’s bodies more broadly—including side effects related to hormones, menstruation, and fertility—read our detailed article on phentermine side effects in females.

Birth Defects and Fetal Development

The available data from animal studies and a limited number of human case reports published in Cureus and other journals suggest a possible link between phentermine use during pregnancy and an increased risk of birth defects, including neural tube defects and major congenital malformations. These defects occur when the fetal neural tube (which becomes the brain and spinal cord) doesn’t form correctly in the first trimester.

Additionally, fetal stroke and restricted fetal growth have been observed in some reports. These complications may affect a child’s long-term health and future behavior.

“We’re particularly concerned about phentermine’s potential to contribute to low birth weight, preterm delivery, and other pregnancy-related problems,” explains Dr. Olesiak. “The risks are not fully quantified, but they are concerning enough to advise against use during pregnancy.”

Preterm Delivery, Gestational Diabetes, and Other Concerns

Some evidence suggests that taking phentermine may increase the risk of pregnancy complications such as preterm delivery, low birth weight, and gestational diabetes. While more published data is needed, these outcomes are considered significant.

Phentermine and Topiramate Combination Risks

The phentermine and topiramate combination drug (Qsymia) raises even more red flags. Topiramate has been directly linked to birth defects, especially oral clefts, in multiple pregnancy registries. Due to this, Qsymia is contraindicated during pregnancy and carries an FDA product label warning against use by women of childbearing potential without effective birth control.

How Long Phentermine Stays in the Body During Pregnancy

Phentermine has a half-life of about 20 hours, but it may take several days to fully clear from the body. The exact time a drug stays in your system depends on metabolism, liver function, and dosage.

However, any potential maternal use near conception or in early pregnancy can pose risks for infants exposed in utero.

Why You Should Stop Taking Phentermine During Pregnancy

Pregnant patients are advised to stop taking the medication altogether and speak with their healthcare provider immediately. Continuing use could not only impact birth weight and fetal development, but also result in other pregnancy complications.

While some pregnant women fear stopping weight loss efforts, it’s important to understand that treating obesity during pregnancy should not include appetite suppressants or stimulants.

What to Do If You Took Phentermine Before Knowing You Were Pregnant

This is a common concern, especially among women in the first and second trimesters who used the drug before a positive test. If this happens, contact your healthcare provider right away. You may need additional screenings to monitor fetal development, including scans to detect potential birth defects.

Remember, a limited number of exposures in early pregnancy don’t guarantee harm. Many babies born after unintentional use are healthy, but medical monitoring is essential.

What Research Says About Phentermine Use During Pregnancy

Despite the widespread use of phentermine to treat obesity in healthy nonpregnant adults, only a limited number of high-quality studies evaluate its safety during pregnancy. Most of the available published literature comes from retrospective reports, case studies, and animal models. This lack of robust clinical data creates a significant gap in understanding how phentermine exposures might influence fetal development and long-term outcomes in pregnant women.

That said, the pharmacologic mechanism of phentermine—as a central nervous system stimulant—raises serious concerns. It may affect fetal growth, increase the risk of fetal stroke, and contribute to major congenital malformations, particularly if taken during the first trimester or early pregnancy. Several medical organizations and expert panels recommend avoiding the drug during pregnancy due to these possible risks, even though direct evidence is still developing.

Until more published data become available, healthcare providers generally err on the side of caution and advise discontinuing phentermine as soon as pregnancy is confirmed.

Why Birth Control Is Essential While Taking Phentermine

Because of the potential for birth defects and other pregnancy-related problems, women of childbearing potential who are taking phentermine are strongly encouraged to use effective birth control. The goal is to prevent inadvertent fetal exposure, particularly during early pregnancy when critical stages of fetal development occur. Even short-term use of the medication around the time of conception could lead to pregnancy complications, including low birth weight, preterm delivery, or neural tube defects.

It’s important for healthcare providers to have proactive conversations about contraception with patients prescribed phentermine or phentermine and topiramate. They should also review the product labels of any combined weight loss medications for warnings related to pregnancy increase in risk.

Additionally, while birth control pills are generally compatible with phentermine, certain combinations—especially those containing topiramate—may reduce the effectiveness of hormonal contraceptives. Patients should disclose all other medications they’re taking and ask about backup contraceptive methods when appropriate.

How Phentermine May Affect Fertility and a Child’s Future Behavior

There is limited data suggesting that stimulants like phentermine may affect fertility, especially in high doses. More concerning is the question of whether pregnancy affects future behavior, whether prenatal exposure to stimulants can alter brain development in ways that impact childhood cognition, learning, or impulse control.

Some animal studies suggest that in-utero stimulant exposure could influence a child’s future behavior, but this remains an area requiring more research.

Safer Ways to Manage Weight During Pregnancy

Managing weight gain during pregnancy is important for both maternal and fetal health—but it must be done safely. Weight loss medications, including appetite suppressants like phentermine, are not approved for use during pregnancy due to potential harm to the developing baby. Instead, pregnant women should work closely with their healthcare provider to adopt safe, evidence-based approaches that support a healthy pregnancy.

Safe alternatives may include:

• Referral to a prenatal dietitian: A registered dietitian specializing in pregnancy nutrition can help create a personalized eating plan that ensures you and your baby get the necessary nutrients without excessive weight gain.
• Modified physical activity plans: If cleared by your healthcare provider, low-impact exercises like walking, swimming, or prenatal yoga can help manage weight and reduce the risk of gestational diabetes or other pregnancy complications.
• Blood sugar monitoring: For women with risk factors such as obesity or a history of insulin resistance, monitoring blood sugar can help prevent or manage gestational diabetes, which is a common concern in pregnancy and can lead to pregnancy-related problems if left untreated.

These strategies aim not for weight loss, but for optimal weight maintenance and prevention of pregnancy complications such as preterm delivery or low birth weight. The focus during pregnancy should be on nourishment, energy, and a safe environment for the baby—not on dieting or rapid fat reduction. Always seek guidance from healthcare providers before making any significant changes to your diet, activity level, or medication regimen.

What to Know About Taking Phentermine While Breastfeeding

Though the current focus is on pregnancy, many women also have breastfeeding questions after delivery. Should you resume taking phentermine while nursing?

The answer is not straightforward. Phentermine may pass into breast milk, and there’s little known about its safety for infants exposed postnatally. This is another area where there’s only a limited number of studies.

Be sure to ask your provider all your breastfeeding questions before restarting weight loss drugs postpartum.

Frequently Asked Questions About Phentermine and Pregnancy

Navigating pregnancy while managing weight or ongoing prescriptions can be overwhelming.

Below are answers to some of the most common concerns about taking phentermine during pregnancy, its effects on birth control, and how long it may remain in your system.

1. What happens if I take phentermine while pregnant?

Taking phentermine while pregnant may increase the risk of serious complications, including birth defects, low birth weight, and preterm delivery. The drug is not approved for use in pregnancy, and most healthcare providers recommend stopping it as soon as pregnancy is confirmed.

If you’ve already taken it during early pregnancy, it’s important to speak with your healthcare provider about additional monitoring. Not all exposures result in harm, but caution is essential given the limited data and potential risks.

2. Can you take weight loss pills while pregnant?

No, weight loss pills—including phentermine alternatives that offer appetite suppression—are not considered safe during pregnancy. These medications are designed for non-pregnant adults and may interfere with fetal development or contribute to pregnancy complications. Managing weight gain during pregnancy should focus on healthy eating and activity, not pharmacologic intervention.

Always consult your healthcare provider before starting or continuing any medications while pregnant.

3. How long does phentermine stay in your system during pregnancy?

Phentermine has a half-life of approximately 20 hours in healthy adults, meaning it may take several days to fully clear from your system. However, metabolism can slow during pregnancy, potentially extending the drug’s presence in the body. This can increase fetal exposure if the medication is taken after conception.

For this reason, phentermine should be discontinued as soon as pregnancy is confirmed or suspected.

4. What does phentermine do to birth control?

Phentermine is not known to reduce the effectiveness of hormonal birth control pills directly. However, it may cause side effects like nausea or vomiting, which can interfere with oral contraceptive absorption if vomiting occurs shortly after taking a pill. Some phentermine-containing products (like phentermine and topiramate) carry warnings due to topiramate’s interaction with estrogen-containing contraceptives.

If you’re using birth control and taking phentermine, discuss your options with your healthcare provider to ensure proper protection.

Final Thoughts

Taking phentermine while pregnant presents more than theoretical risks. While some women have used the medication early in pregnancy without known consequences, the lack of robust data combined with potential harms—including birth defects, fetal stroke, low birth weight, and other pregnancy-related problems—makes discontinuation the safest course.

Ultimately, the best approach involves early discussions with your healthcare provider, clear planning around birth control, and a focus on health over weight loss during pregnancy.

References

Cureus

FDA