The following table lists pharmaceuticals that have been available in both racemic and single-enantiomer form. These single-enantiomer drug switched from the respective racemic drug are referred to as chiral switch.
The following are cases where the individual enantiomers have markedly different effects:
*Thalidomide: Thalidomide is racemic. One enantiomer is effective against morning sickness, whereas the other is teratogenic. However, the enantiomers are converted into each other in vivo. As a result, dosing with a single-enantiomer form of the drug will still lead to both the enantiomers eventually being present in the patients serum and thus would not prevent adverse effects—at best, it might reduce them if the rate of in vivo' conversion can be slowed.
*Ethambutol: Whereas the (S,S)-(+)-enantiomer is used to treat tuberculosis, the (R,R)-(–)-ethambutol may cause blindness.
*Steroid receptor sites also show stereoisomer specificity.
*Penicillin's activity is stereodependent. The antibiotic must mimic the -alanine chains that occur in the cell walls of bacteria in order to react with and subsequently inhibit bacterial transpeptidase enzyme.
*Propranolol: -propranolol is a powerful adrenoceptor antagonist, whereas -propranolol is not. However, both have local anesthetic effect.
*Methorphan: The -isomer of methorphan, levomethorphan, is a potent opioid analgesic, while the -isomer, dextromethorphan, is a dissociative cough suppressant.
*Carvedilol: (S)-(–)-isomer interacts with adrenoceptors with 100 times greater potency as β adrenoreceptor blocker than (R)-(+)-isomer. However, both the isomers are approximately equipotent as α adrenoreceptor blockers.
*Amphetamine and methamphetamine: The -isomers of these drugs are strong central nervous system (CNS) stimulants, while the -isomers lack appreciable CNS stimulant effects, but instead stimulate the peripheral nervous system. For this reason, the -isomer of methamphetamine is available as an over-the-counter nasal inhaler in some countries, while the -isomer is banned from medical use in all but a few countries in the world, and highly regulated in those countries which do allow it to be used medically.
*Ketamine: This drug is available as a mixture of both (S)-(+)-ketamine, also known as esketamine, and (R)-(–)-ketamine, also known as arketamine. Pure esketamine is also available. The two have different dissociative and hallucinogenic properties, with esketamine being more potent in isolation as a dissociative. The two enantiomers have inverse effects on the rate of glucose metabolism in the frontal cortex.
*Dihydroxy-3, 4 phenylalanine (Dopa): Dopa is a racemic mixture where one enantiomer, L-Dopa, is used as a treatment for Parkinson's Disease, and the other enantiomer, D-Dopa is considered to be toxic. D-Dopa can cause headaches, abdominal pains, nausea, vomiting, and dizziness.