On October 1, 1957, a new over-the-counter drug was introduced in West Germany that was supposed to basically cure morning sickness during pregnancy.

It was called thalidomide. Thousands of women purchased the drug and were relieved when their morning sickness disappeared.

And everything seemed great until later the next year, when thousands of infants were born with severe birth defects, like brain damage and deformed limbs. Doctors noticed the surge, but at first they couldn’t figure out what was causing it. Then, two doctors, Widukind Lenz from Germany and William McBride from Australia, both noticed that the mothers of these babies had all taken that amazing new morning sickness drug.

By 1961, Lenz had proven the link between thalidomide and the birth defects, and it was pulled from the shelves. But by then, thousands of infants had been born with those birth defects, and nearly 40% of them had passed away. Rules for drug testing weren’t as strict back then, and in this case, that cost thousands of lives.

Even though they were now aware that thalidomide was a problem, scientists still didn’t know why. Turns out the answer lies in its chemistry — or more specifically, its geometry. What researchers didn’t know at the time was that there were actually two versions of the thalidomide molecule.

Thalidomide, which is made up of three connected rings of carbon and nitrogen atoms, plus some oxygen and hydrogen, is what’s known as a chiral molecule. Chiral comes from the Greek for hand or claw, and chemists often think of chiral molecules as being right-handed or left-handed. And that’s because the different versions of chiral molecules — called enantiomers — are made up of all the same parts, but arranged differently.

Just like your hands. If you try overlaying your right hand on your left, you’ll see that no matter which way you turn your hands, you can’t make them match up. Even if you make your thumbs line up, one hand is palm up and the other is palm down.

In other words, both hands have the same five fingers, but they’re not identical. And molecules like thalidomide are the same way. A chiral molecule has an atom — typically carbon — with four different groups attached to it.

That’s called the chiral center. A group can be anything from a single atom, to a long chain, to rings. In some cases, two groups can even be connected.

These groups are kind of like your fingers: in each enantiomer, the groups are attached to the chiral center in the opposite order, so they’re mirror images of each other. In thalidomide, for example, the chiral center is a carbon atom attached to one of the rings. It’s bonded to two other carbons, a nitrogen, and a hydrogen.

If you position the version of the molecule that’s considered “right-handed”, called the R enantiomer, so the closest oxygen atom to the chiral center is below it, the hydrogen atom attached to the center will be facing away from you. But if you position the “left-handed”, or S enantiomer, the same way, the hydrogen atom will be facing toward you. In other words, the two enantiomers are mirror images of each other.

And because different receptors and enzymes inside your body react with molecules in very specific ways, the fact that enantiomers are mirror images means that they might react to things totally differently. In the case of thalidomide, scientists discovered that the R enantiomer helped with morning sickness, but the S enantiomer caused those severe birth defects. One of the most famous — or maybe infamous — examples of how two enantiomers affect the human body is methamphetamine.

You’d probably associate that name with the street drug, but the molecule can only really break bad when it’s in its R formation. The S enantiomer is commonly sold over the counter as a vasodilator, and you can easily find it in vapor inhalers. But the vapor inhaler you just bought at the pharmacy isn’t filled with a dangerous drug.

Because the S version will never be psychoactive. After researchers discovered that only the S version of thalidomide caused birth defects, they thought about isolating the R enantiomer so they could continue to use it to treat morning sickness. But it turns out that thalidomide’s R enantiomer can actually switch to the S version while inside the human body.

Meaning that even if they’d isolated 100% pure R thalidomide, it still wouldn’t be safe for pregnant women. But in 1998, the FDA did approve thalidomide, almost forty years after rejecting it based on the birth defect problems. This time, though, it was approved for treating complications from Hansen’s Disease, aka leprosy.

Recent research is also leading some scientists to believe that in certain cases, thalidomide could help with a number of debilitating diseases, including some forms of breast cancer. They just make sure not to prescribe it to anyone who’s pregnant. Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon.

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