Stereoisomer

A stereoisomer is a molecule that shares the same atoms and the same sequence of bonds as another molecule, but arranges those atoms differently in three-dimensional space. Two stereoisomers are chemically identical on paper yet physically distinct objects — the most familiar case being enantiomers: mirror-image pairs that cannot be superimposed, much like a left and right hand.

The property that produces enantiomers is called chirality. A carbon atom bonded to four different groups is a chiral centre, and its two possible arrangements are labelled R and S by convention. Many drugs contain one or more chirality centres, making stereoisomerism central to how pharmacologists understand and design them.

A second category, diastereomers, covers stereoisomers that are not mirror images — including cis and trans geometric isomers. Diastereomers differ more broadly in their physical properties than enantiomers do.

How it works · its role

Biological systems are themselves chiral: proteins, enzymes, and receptors are built from amino acids that almost universally adopt one spatial orientation. Because of this, a receptor's binding pocket is shaped to favour one stereoisomeric form of a molecule over its mirror image.

The two enantiomers of a drug can therefore behave quite differently in the body. One form may bind tightly to its target receptor while its mirror image binds weakly, not at all, or even to a completely different target. They may also be metabolised at different rates, producing different durations of effect or different breakdown products.

This is why pharmaceutical development often involves chiral switching: isolating a single enantiomer from a previously racemic mixture (50/50 mixture) drug to improve potency, reduce side effects, or extend patent protection.

Relevance to substances & effects

Stereoisomerism shapes the pharmacology of a wide range of psychoactive substances. Amphetamine is sold as a racemic mixture and as the isolated S-enantiomer (levoamphetamine) or R-enantiomer; the two differ meaningfully in their ratio of central to peripheral stimulant effects.

Ketamine provides a well-studied example: the S-enantiomer (esketamine) binds the NMDA with roughly twice the affinity of the R-form, producing stronger dissociative and anaesthetic effects at lower doses. The R-form is thought to contribute more to certain residual or emerging therapeutic effects — an active area of research.

Classic psychedelics also illustrate the principle. Among the DOx and NBOMe families, differences between stereoisomers at specific chiral centres are associated with large changes in receptor affinity and potency. Even where a drug lacks a conventional chiral centre, geometric isomerism (cis/trans) can determine whether a compound is active at all.

In practice, when a substance page lists a specific enantiomer or notes that a compound is racemic, it is describing which stereoisomeric form is present — information that directly affects expected potency, onset, and effect profile.