"Many drugs work by a molecule of the active ingredient binding with a receptor or the active site of an enzyme. Because receptors have a 3D structure, only one of a pair of optical isomers will fit. In some cases, one optical isomer is an effective drug and the other is inactive"
If only one enantiomer can bind with the receptor, how do some drugs have both enantiomers taking effect? E.g. in thalidomide both the S-form which caused birth defects and the R from which was a safe sedative took effect? Or was it only the S-from that was active? I'm confused.