What Drug Causes Severe Birth Defects?

Medically Reviewed on 3/29/2022
What drug causes severe birth defects
Thalidomide is a potent teratogenic drug, which is an agent known to cause abnormalities in the fetus following exposure during pregnancy.

Thalidomide is a potent teratogenic drug (an agent that causes abnormalities in the fetus following its exposure during pregnancy).

  • Thalidomide is the earliest known drug that causes severe life-threatening birth defects.
  • Earlier in the 1950s and 1960s, thalidomide was used to cure morning sickness in pregnant women. It was prescribed for pneumonia and cold-like symptoms.
  • Later, it was revealed that women who were on thalidomide during pregnancy gave birth to children with severe disabilities, most commonly limb deformations, blindness, heart defects, deafness, and brain damage. Thalidomide even caused the death of the unborn child.


The U.S. Food and Drug Administration has approved a program called THALOMID (thalidomide) Risk Evaluation and Mitigation Strategy (REMS) which was earlier known as the System for Thalidomide Education and Prescribing Safety (STEPS). This program aims to ensure that pregnant women do not use thalidomide, and women should not become pregnant while on thalidomide.

The guidelines for THALOMID REMS are as follows:

  • All men and women who are on prescription thalidomide and who find it difficult to get pregnant must be registered with THALOMID REMS.
  • Thalidomide should be prescribed only by a doctor who is registered with THALOMID REMS.
  • Prescription thalidomide should be refilled by a pharmacy that is registered with THALOMID REMS.

What are the other potential teratogenic drugs?

Apart from thalidomide, many drugs were found to be teratogenic, such as:

All the drugs mentioned above are useful to treat various serious medical conditions. These drugs should be taken cautiously in women who are planning for pregnancy or who are pregnant. Some medications for chronic diseases such as asthma, high blood pressure, or epilepsy may not be avoidable during pregnancy, so close observation by a doctor while taking these medications during pregnancy is highly recommended.

The list of teratogenic drugs is not confined to this. Many other drugs are known to cause birth defects.


Conception: The Amazing Journey from Egg to Embryo See Slideshow

Evidence-based grading system for teratogenic drugs

The U.S. Food and Drug Administration uses an evidence-based grading system as part of product labeling to grade drugs based on their potential for teratogenic risk. Drugs in group A have no demonstrated danger, whereas those in categories D and X have a definite risk and positive evidence of fetal harm.

Classification includes:

  • A: Prescription drugs that have been used by a large number of pregnant women with no evidence of an increased risk of birth abnormality.
  • B: Drugs that have only been used by a small number of pregnant women. There is a shortage of human data; hence, they are further classified based on existing data from animal studies.
  • B1: Animal studies have reported no increased risk.
  • B2: Although animal studies are limited, there does not appear to be an elevated risk.
  • B3: Animal studies report an increased risk although it is unclear whether these drugs are dangerous to humans.
  • C: These drugs may harm the fetus while not causing birth abnormalities. These effects could be reversed.
  • D: Drugs that have caused or are likely to cause birth abnormalities; however, the health benefits may outweigh the risks.
  • X: Drugs that are associated with a high risk of birth abnormalities and should not be used during pregnancy.

Drugs that come under category D must be used only if:

  • The mother is at high risk in the absence of the drug.
  • The advantage of drugs is higher than the risk it poses.

Drugs that come under category X must be avoided completely in a pregnant mother because they cause severe birth defects.

A safer medicine can be substituted with harmful drugs to treat certain medical conditions during pregnancy such as:

  • Heparin can be used instead of warfarin to prevent blood clots.
  • Penicillin is a safer antibiotic that can be used to treat infections during pregnancy.

How teratogenic effects are caused by drugs in the fetus?

Most of these drugs taken by the pregnant mother cross the placenta and produce harmful effects on the fetus. Although some drugs may not cross the placenta, they still harm the fetus by affecting the placenta or uterus.

  • They can have an immediate effect on the fetus, causing harm, which results in birth abnormalities or death.
  • They can affect the function of the placenta, mainly by causing blood vessels to narrow (constrict) and limiting the delivery of oxygen and nutrients from the mother to the fetus. As a result, the newborn may be underweight and undeveloped.
  • They can cause the uterine muscles to contract violently, harming the fetus indirectly by limiting its blood supply or prompting premature labor and delivery.
  • They can have an indirect effect on the fetus. Drugs that lower the mother's blood pressure may restrict blood flow to the placenta, reducing the fetus's availability of oxygen and nutrients.

The teratogenic effect of a drug depends on various factors such as:

  • The developmental stage of the fetus
  • The drug's potency and dosage
  • The extent to which the drug can cross the placenta
  • The mother's genetic makeup influences how much of the medicine is active and available

Are birth defects diagnosed before birth?

It is not always possible to detect all birth abnormalities while the baby is still in the womb. Many birth defects, however, may be diagnosed through various tests including high-resolution ultrasounds and genetic tests. 

The only preventive measure is to avoid teratogenic drugs before or during pregnancy and quit consumption of alcohol, tobacco, and other illicit drugs.

Medically Reviewed on 3/29/2022
Image Source: iStock Images

Teratology and Drug Use During Pregnancy: https://emedicine.medscape.com/article/260725-overview#a2

Teratogens: https://www.sciencedirect.com/topics/neuroscience/teratogens

Birth Defects: https://my.clevelandclinic.org/health/diseases/12230-birth-defects