and levocetirizine is a chiral switch of (±)-cetirizine. While often only one enantiomer of the drug may be active, there are cases in which the other enantiomer is harmful, like salbutamol and thalidomide. The (R) enantiomer of thalidomide is effective against morning sickness, while the (S) enantiomer is teratogenic, causing birth defects. Since the drug racemizes, the drug cannot be considered safe for use by women of child-bearing age, and its use is tightly controlled when used for treating other illness. Methamphetamine is available by prescription under the brand name Desoxyn. The active component of Desoxyn is dextromethamphetamine hydrochloride. This is the right-handed isomer of methamphetamine. The left-handed isomer of methamphetamine, levomethamphetamine, is an OTC drug that is less centrally-acting and more peripherally-acting. Wallach's rule Wallach's rule (first proposed by Otto Wallach) states that racemic crystals tend to be denser than their chiral counterparts. This rule has been substantiated by crystallographic database analysis. See also Chiral switch Chirality (same as optical isomerism) Descriptor (chemistry) Racemic (protein) crystallography Racemization Skeletal formula#Stereochemistry which describes how stereochemistry is denoted in skeletal formulae References Stereochemistry Chemical nomenclature Organic chemistry