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What Effects Does Phentermine Have on Pregnancy

author image Roxanne Maas
Based in Los Angeles, Roxanne Maas holds a master's degree in genetic counseling with more than 12 years of experience communicating complex genetics and pregnancy information. She has published several abstracts in scientific journals, presented posters at national genetics meetings, and published health-related articles on LIVESTRONG.com.
What Effects Does Phentermine Have on Pregnancy
Limited information is available on the safety of phentermine during pregnancy. Photo Credit TongRo Images/TongRo Images/Getty Images


Phentermine is a stimulant used as an appetite suppressant. In combination with diet and exercise, phentermine can help people lose weight. Until 1997, phentermine was also sold in combination with fenfluramine under the brand name Fen Phen. While many women are concerned about the weight gain associated with pregnancy, doctors caution against trying to lose weight during pregnancy.

Risks for Mother

The Drugs website recommends that phentermine be used for only a short period of time, such as a few weeks. Long-term use of high doses of phentermine can cause severe skin problems, sleep problems including insomnia, personality changes, hyperactivity or irritable feelings. Phentermine used in combination with other diet medications such as fenfluramine can cause a rare fatal lung disorder called pulmonary hypertension. Therefore, women using phentermine should speak with their doctor before taking any additional diet medications.

Risks for Baby

Little information is available regarding risks to babies exposed to phentermine during pregnancy. However, two available reports do not suggest an increased risk for birth defects or other adverse outcome. A preliminary report on 100 women who used Fen Phen in early pregnancy by K.A. Johnson published in the journal “Teratology” in 1998 did not find an increased incidence of physical abnormalities. Another study, by K.L. Jones and published in “Teratology” in 2002, did not find an increased incidence of major congenital anomalies in 84 infants of women who used Fen Phen during the first trimester of pregnancy. Because these reports include small numbers of exposed pregnancies, they cannot definitively rule out risks for adverse outcomes in babies exposed to phentermine. Pregnant women are encouraged to discuss diet and weight loss options with their doctor before starting on a program.

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Risks of Weight Loss

Although some problematic medical conditions, such as gestational diabetes and high blood pressure, are more likely during pregnancy when women are overweight, it is not recommended that women lose weight during pregnancy, as recommended by the Organization of Teratology Information Services. The growing fetus needs sufficient nutrition to grow properly, and dieting can rob the baby of necessary nutrients.

Side Effects

Various side effects have been associated with phentermine, including dry mouth, unpleasant taste, diarrhea, constipation and vomiting. Medline Plus explains that these symptoms can be largely avoided by taking phentermine with food. Severe side effects that warrant an immediate call to the doctor include increased blood pressure, heart palpitations, dizziness, tremor, shortness of breath and chest pain.

Withdrawal Symptoms

The Drugs website warns that withdrawal symptoms, such as depression and extreme tiredness, may occur when phentermine is discontinued after a long period of use. Phentermine should be taken and discontinued under a doctor’s supervision. In many cases, a doctor will recommend using less and less phentermine before stopping it completely to reduce the chance of withdrawal symptoms.

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  • "Teratology"; Pregnancy Outcome of Women Exposed to Fen/Phen; K.A. Johnson et al; May 1998
  • "Teratology"; Pregnancy Outcomes After First Trimester Exposure to Phentermine/Fenfluramine; K.L. Jones; March 2002
  • Medline Plus: Phentermine
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