Thalidomide is one of the most well-documented teratogens in medical history. A teratogen is any substance that causes birth defects when exposure occurs during pregnancy, and thalidomide’s ability to do so was proven tragically in the late 1950s and early 1960s, when thousands of children were born with severe malformations after their mothers took the drug for morning sickness. Today, thalidomide is still used medically for certain conditions, but under some of the strictest prescribing controls of any drug on the market.
What Thalidomide Does to a Developing Embryo
The damage thalidomide causes depends almost entirely on when during pregnancy a woman is exposed. The critical window runs from day 20 to day 36 after fertilization, which corresponds roughly to days 34 through 50 after the last menstrual period. Exposure before or after this window does not appear to cause harm to the embryo.
Earlier exposure within that window produces the most severe effects. The upper limbs begin forming around day 26 and the lower limbs around day 27, which is why limb defects are the hallmark of thalidomide exposure. The most recognizable pattern is phocomelia, where hands or feet attach almost directly to the shoulders or hips because the long bones of the arms or legs fail to develop. In some cases, entire limbs are missing. Exposure later in the sensitive window tends to affect the fingers and toes, eyes, and ears.
Limbs are not the only structures at risk. Thalidomide exposure during this period can also damage the heart, kidneys, gastrointestinal tract, genitalia, and facial structures. Ear abnormalities range from small, underdeveloped ears to their complete absence. Eye defects include abnormally small eyes, missing eyes, and misaligned eyes. Facial birthmarks called hemangiomas are also common in affected children.
How It Causes Birth Defects at the Molecular Level
For decades, no one understood exactly how thalidomide disrupted fetal development. Researchers eventually identified a protein called cereblon as the drug’s primary target. Cereblon is part of the cell’s protein-recycling machinery. It tags other proteins for destruction, a normal housekeeping function that keeps cells running properly.
When thalidomide binds to cereblon, it changes the shape of the protein’s binding surface. This causes cereblon to start tagging proteins it would normally leave alone, marking them for destruction. Among the downstream effects, thalidomide suppresses a growth signal called FGF8 that is essential for limb and fin development. Without adequate FGF8, limb buds in the embryo fail to grow normally.
This mechanism has been confirmed across multiple species. When researchers engineered a version of cereblon that could not bind thalidomide, zebrafish embryos and chick embryos exposed to the drug developed normally, with no limb defects. The pathway appears to be conserved across evolutionary lines, which explains why the drug causes similar malformations in humans, rabbits, chicks, and zebrafish.
The Thalidomide Disaster of the 1950s and 1960s
Thalidomide was first marketed in the late 1950s, primarily in West Germany, as a sedative and treatment for nausea in pregnant women. It was sold in dozens of countries and widely considered safe. By the early 1960s, physicians in Germany and Australia began noticing clusters of babies born with an extremely rare pattern of limb defects. The connection to thalidomide was established in 1961, and the drug was pulled from most markets.
Thousands of children worldwide were born with thalidomide-related birth defects. Many did not survive infancy due to severe internal organ damage. Those who survived have faced lifelong physical challenges. In the UK, the Thalidomide Trust continues to support 467 beneficiaries, many of whom are now in their 60s and dealing with the compounding effects of aging on bodies that were already structurally compromised.
The United States largely avoided the disaster. Frances Oldham Kelsey, a medical reviewer at the FDA, refused to approve the drug despite sustained pressure from its manufacturer. She insisted on scientifically reliable safety evidence that the company could not provide. Roughly a year after her initial refusal, the link to birth defects became public. The near-miss helped pass the 1962 Drug Amendments, which fundamentally changed how drugs are tested and approved in the U.S. and established the requirement that manufacturers prove both safety and effectiveness before a drug reaches the market.
Why Thalidomide Is Still Prescribed Today
Despite its history, thalidomide turned out to have genuine therapeutic value. It is currently approved for two conditions: multiple myeloma (a blood cancer), where it is used in combination with other medications, and erythema nodosum leprosum, a painful inflammatory complication of leprosy. Its ability to modify the immune system and inhibit the growth of new blood vessels makes it effective for these conditions even though the same properties are what make it dangerous to a developing embryo.
Two related drugs, lenalidomide and pomalidomide, were developed as modified versions of thalidomide. Both are also effective treatments for multiple myeloma, and both carry the same teratogenic risk.
How Pregnancy Exposure Is Prevented
Thalidomide can only be prescribed and dispensed through a mandatory safety program called THALOMID REMS. Only certified prescribers and pharmacies can handle the drug, and every patient must enroll in a confidential surveillance registry.
The requirements for women of childbearing potential are particularly stringent. Two forms of birth control must be used starting at least four weeks before treatment begins and continuing for at least four weeks after stopping the drug. Pregnancy testing is required within 24 hours before the first prescription, then weekly for the first month, and every two to four weeks after that depending on menstrual regularity. A monthly telephone survey must be completed before each new prescription can be written. If a pregnancy occurs, the drug is stopped immediately and the case is reported to both the FDA and the manufacturer. Lenalidomide and pomalidomide operate under similar mandatory safety programs with the same core pregnancy prevention requirements.
These controls have made thalidomide-related birth defects exceedingly rare in countries where the programs are enforced, though the drug’s teratogenic potency has not changed. A single dose during the critical window of pregnancy can cause severe malformations, which is why the safety infrastructure around it remains among the most restrictive for any approved medication.