studies of approximately 8.3 hours. Its duration of action is at least 24 hours. The elimination half-life of cetirizine is increased in the elderly (to 12 hours), in hepatic impairment (to 14 hours), and in renal impairment (to 20 hours). Chemistry Cetirizine contains L- and D-stereoisomers. Chemically, levocetirizine is the active L-enantiomer of cetirizine. The drug is a member of the diphenylmethylpiperazine group of antihistamines. Analogues include cyclizine and hydroxyzine. Synthesis The 1-(4-chlorophenylmethyl)-piperazine is alkylated with methyl (2-chloroethoxy)-acetate in the presence of sodium carbonate and xylene solvent to produce the Sn2 substitution product in 28% yield. Saponification of the acetate ester is done by refluxing with potassium hydroxide in absolute ethanol to afford a 56% yield of the potassium salt intermediate. This is then hydrolyzed with aqueous HCl and extracted to give an 81% yield of the carboxylic acid product. Availability Cetirizine is available without a prescription. In some countries it is available over-the-counter only in packages containing seven or ten 10 mg doses. Cetirizine is available as a combination medication with pseudoephedrine, a decongestant. The combination is often marketed using the same brand name as the cetirizine with a "-D" suffix (for example, Zyrtec-D) Cetirizine is marketed under the brand names Alatrol, Alerid, Alzene, Cerchio, Cetirin, Cetizin, Cetriz, Cetzine, Cezin, Cetgel, Cirrus, Histec, Histazine, Humex, Letizen, Okacet (Cipla), Piriteze, Reactine, Razene, Rigix, Sensahist (Oethmann, South Africa), Triz, Zetop, Zirtec, Zirtek, Zodac, Zyllergy, Zynor, Zyrlek, and Zyrtec (Johnson & Johnson), among others. References External links Belgian inventions Carboxylic acids Ethers H1 receptor antagonists Johnson & Johnson brands Peripherally selective drugs Pfizer brands Chlorcyclizines World Health Organization essential medicines Wikipedia medicine articles ready to translate