tapeworms, including the scolices (plural of "scolex"), are very rarely passed after administration of praziquantel. In many instances, only disintegrated and partially digested pieces of tapeworms will be seen in the stool. The majority of tapeworms are digested and are not found in the feces." Praziquantel is administered as a racemate, but only the (R)-enantiomer is biologically active; the enantiomers may be separated using a resolution of an amine obtained from praziquantel. Pharmacokinetics Praziquantel is well absorbed (about 80%) from the gastrointestinal tract. However, due to extensive first-pass metabolism, only a relatively small amount enters systemic circulation. Praziquantel has a serum half-life of 0.8 to 1.5 hours in adults with normal renal and liver function. Metabolites have a half-life of 4 to 5 hours. In patients with significantly impaired liver function (Child-Pugh score B and C), the serum half-life is increased to 3 to 8 hours. Praziquantel and its metabolites are mainly excreted renally; within 24 h after a single oral dose, 70 to 80% is found in urine, but less than 0.1% as the unchanged drug. Praziquantel is metabolized through the cytochrome P450 pathway via CYP3A4. Agents that induce or inhibit CYP3A4 such as phenytoin, rifampin, and azole antifungals will affect the metabolism of praziquantel. Praziquantel has a particularly dramatic effect on patients with schistosomiasis. Studies of those treated have shown that within six months of receiving a dose of praziquantel, up to 90% of the damage done to internal organs due to schistosomiasis infection can be reversed. History Praziquantel was developed in the laboratories for parasitological research of Bayer AG and Merck KGaA in Germany (Elberfeld and Darmstadt) in the mid-1970s. Society and culture Brand names Biltricide (Bayer) Tablets (for human use) Cesol (Merck) Tablets Cestoved (Vedco) both tablets and injectable for veterinary use Cysticide (Merck) Tablets Distocide (Shin Poong Pharm. Co., Ltd.) tablet (for human use) Distoside (Chandra Bhagat Pharma Pvt Ltd) tablet (for human use) Droncit (Bayer) for veterinary use Drontal (combination with pyrantel pamoate) (Bayer) for veterinary use D-Worm (Farnum) for veterinary use; note that D-Worm also makes roundworm medicine containing piperidine which is not effective against tapeworms. Fish Tapes (Thomas Labs) for aquarium use Interceptor Plus chewable tablets (combination with milbemycin) (Elanco) for veterinary use. Note regular Interceptor only has milbemycin and does not contain praziquantel. Kaicide (Taiwan) Milbemax (combination with milbemycin oxime) (Novartis) for veterinary use Popantel (Jurox) PraziPro (Hikari) for aquarium use