Once you accept that mirror-image forms can behave differently, a practical question follows for anyone making medicines. Should a drug contain just one form, or both?

Some medicines are built from a single selected form. Esomeprazole, a common heartburn and acid reflux medicine used to reduce stomach acid, contains one specific form of omeprazole. Levosalbutamol, a breathing medicine for asthma, contains one selected form of salbutamol. Choosing a single form lets scientists concentrate on the exact molecular shape that contributes most to the intended effect.

Other medicines contain both mirror-image forms together, the left-handed and the right-handed versions in one product. This is called a racemic mixture. Ordinary omeprazole is a racemic product, containing both forms, while esomeprazole narrows this down to mainly one. When both forms are present, researchers have to ask whether both contribute to the treatment, or whether one does most of the work while the other sits weaker, inactive, or behaving differently.

The two forms of a drug can differ in how active they are, how safe they are, and how the body absorbs, distributes, breaks down, and clears them. No episode in the history of medicine made this clearer, or more painfully, than thalidomide.

In the late 1950s thalidomide was sold across Europe and beyond as a remedy for morning sickness in pregnant women. Like salbutamol, it was a racemic mixture of two mirror-image forms, and at the time pharmaceutical science did not yet grasp how much a molecule's handedness could change its behaviour in the body. The two forms of thalidomide turned out to behave very differently. The R-form acts broadly as intended, binding in the central nervous system to induce sleep and calm nausea. The S-form interacts differently and can cause severe developmental harm, fitting into a protein called cereblon and disrupting the normal development of a growing foetus.

_{The consequences were devastating. A wide range of birth defects followed, affecting limbs, ears, eyes, internal organs, the face, and the central nervous system. The most recognised of these was phocomelia, in which limbs failed to develop properly and were left underdeveloped or missing. Within a few years of widespread use across Europe, Australia, and Japan, roughly ten thousand children were born with thalidomide-associated defects, and the drug was withdrawn from most countries in 1961.}

The obvious response, once the S-form was identified as the harmful one, was to remove it and give patients only the safe R-form. With many chiral drugs this approach works cleanly. Thalidomide, however, undergoes rapid racemisation inside the body.

Under the body's conditions, the hydrogen at the molecule's chiral centre can be removed, forming an intermediate that has lost its handedness, and when the hydrogen returns either form can be regenerated. The practical result is that giving a pure single form does not keep it pure. The body reconstitutes both forms regardless, so exposure to the harmful form cannot be avoided by separation alone. This is exactly why the story cannot be reduced to one good form and one bad form. Stereochemistry, metabolism, toxicity, and biological behaviour all have to be studied together.

The tragedy reshaped how the world develops drugs. It pushed regulators, including the FDA, to rewrite the rules, and from that point drug companies were required to study each enantiomer of a chiral drug and show how both forms behave before a product could reach the market. The molecular handedness that had once been overlooked became something every new medicine had to account for.

Thalidomide itself was later reintroduced under tightly controlled conditions, after it was found to restrict the blood supply that tumours depend on, and it is used today in the treatment of leprosy complications and multiple myeloma, always with strict safeguards to keep it away from pregnancy.