Phentermine is in a class of drugs called anorectics. These drugs help to suppress appetite and promote weight loss.

Phentermine (Adipex-P, Lomaira) is a prescription oral medication. It’s also available as a combination with another drug called topiramate, marketed as Qsymia.

Phentermine is used temporarily in people who are overweight or obese and are actively trying to lose weight through diet and exercise. It’s temporary because its effectiveness decreases after three to six weeks.

Phentermine acts much like a stimulant and has many of the same side effects:

  • increased heart palpitations
  • increased blood pressure
  • dizziness

Phentermine is known for being one part of Fen-Phen, a weight loss medication that also included the drug fenfluramine. Fen-Phen was taken off the market by the U.S. Food and Drug Administration (FDA) in 1997 after safety concerns surrounding fenfluramine.

Phentermine alone, however, has been used for decades and appears to be a safe and effective weight loss medication when used in the short-term in healthy individuals.

One study found that phentermine significantly reduced weight without raising blood pressure or causing other heart problems. Some patients lost more than 10 percent of their body weight and were able to keep the pounds off for eight years.

However, very little is known about the effects of phentermine on pregnant women or unborn babies during pregnancy. As such, it’s not approved for use during pregnancy. Phentermine and other appetite suppressants are typically not recommended during pregnancy because most women shouldn’t lose weight while they’re pregnant.

If you took phentermine before pregnancy or before you knew you were pregnant, you may have concerns about its effects on your developing baby. Let’s take a closer look at what you should know.

If you took phentermine before pregnancy, it should have no effect on your ability to carry a healthy baby to term. All traces of phentermine should pass through your body. Even if you took your last dose a week before conception, it shouldn’t have any effect on your pregnancy.

Very few human or animal studies have been done on phentermine during pregnancy. But the very few that do exist don’t seem to connect the drug to birth defects.

One very small study compared pregnant women in the Czech Republic who took phentermine or sibutramine, another appetite suppressant, to pregnant women who didn’t take the drugs. No differences in pregnancy outcomes were found.

While research about pregnancy and phentermine on its own is lacking, another study looked at the use of phentermine/fenfluramine, which is no longer available, in the first trimester of pregnancy. It showed that, when compared with women who didn’t use the drug, women who used it had no greater risk of:

  • miscarriage
  • preterm delivery
  • babies with birth defects

Qsymia is considered a category X drug by the FDA. That means the drug has the potential to cause birth defects and shouldn’t be used during pregnancy. Research indicates that the topiramate contained in the drug can increase the risk for cleft lip in babies.

Again, little is known about phentermine use and its effect on a developing baby or pregnant women. One study from 2002 does point to a higher risk of gestational diabetes among pregnant women who took phentermine/fenfluramine during the first trimester. But this increased risk of gestational diabetes was probably related to being overweight to begin with, rather than a side effect of the drug.

Gestational diabetes can increase the risk for a number of health complications for pregnant women, including:

  • giving birth to a large baby, which can cause delivery complications
  • high blood pressure and preeclampsia, which can be potentially life-threatening
  • adult-onset diabetes later in life

Although weight loss during pregnancy isn’t generally recommended, research found that 8 percent of pregnant women attempt it. While phentermine wasn’t part of this study, phentermine is connected to weight loss.

The American College of Obstetricians and Gynecologists suggests a weight gain during pregnancy of:

  • 25 to 35 pounds for women not overweight
  • 15 to 25 pounds for women who are overweight
  • 11 to 20 pounds for women who are obese

Weight loss during pregnancy — or not gaining the appropriate amount of weight — can put your baby at risk for a variety of health complications, including:

  • Being small for its gestational age. This increases chances of:
    • trouble maintaining body temperature
    • low blood sugar, which can make a baby sluggish
    • difficulty breathing
  • Dying in the first year of life. In one study, babies born to women who didn’t gain an adequate amount of weight during pregnancy had three times the risk of dying in their first year of life compared with babies born to women who gained an appropriate amount of weight.
  • Disabilities. According to the Office on Women’s Health, pregnant women who restrict their calories to the point that their fat stores are broken down and ketones develop are at risk of delivering children with mental deficiencies.
  • Neural tube defects. Research suggests that using weight loss products during pregnancy can increase the risk for giving birth to a baby with this defect that affects the brain and spine.

It’s possible for phentermine to be excreted in breast milk. For that reason, it’s not recommended for women who are breastfeeding.

Like a lot of things with phentermine, how it affects a breastfed baby hasn’t been well studied. However, because it acts as a stimulant, it may cause side effects like agitation and sleeping and feeding problems.

Studies surrounding phentermine use in pregnant and breastfeeding women are sparse at best.

If you’re using phentermine and are pregnant or nursing, the safest course is to stop immediately. Your doctor can help you assess any potential risks and give you advice on weight gain and management before, during, and after pregnancy.