Abstract
The stereochemistry is important to drug action because the shape of the drug molecule is an important factor in determining how it interacts with various biological molecules (Enzymes – Receptors… etc.) e.g. dextromethorphan and levomethorphan, the dextromethorphan is antitussive and levomethorphan is opioid due to this difference of pharmacological activity is due to stereochemistry of methorphan (Dextro – Levo), cisplatin and transplatin, cisplatin is anti cancer, which is more active than transplatin, trans diethylstilbestrol and cis diethylstilbestrol, the trans form is more active than cis form, escitalopram is pure enantiomer for treat anxiety and depression, which is more active and more fast than citalopram which is mixture of two enantiomers of citalopram, cetirizine and levocetirizine are antihistaminic but the levo cetirizine is more safe and faster action than cetirizine, erythro ephedrine and threo ephedrine which is used in common rhinitis disease under names pseudo ephedrine, levodopa and dopa the levodopa used in treatment of parkinsonism, which is more active than dopa, levothyroxine and dextrothyroxine the levothyroxine used in hypothyroidism but the dextro inactive or less active, and levetiracetam which is levo enantiomer which used in treatment of epilepsy, the levo-enantiomer is more active than dextro enantiomer.