While the newer short-course cephalosporins are only available in brand-name form and are much more expensive, older ones such as cephalexin (better known as Keflex) cost about the same as penicillin or amoxicillin but are more effective against strep.
Nearly all drugs fail some of the time, Pichichero said, and doctors accept some risk of failure as a trade-off for other factors like convenience, cost to families, and likelihood that a less powerful medication can kill the bug just as well as a more powerful drug. In the case of throat infections, doctors try to preserve their strongest antibiotics for the most severe cases. But most doctors would view a drug that fails in one out of four patients, as penicillin does in strep cases, as unacceptable, Pichichero said.
To understand why the older drugs don't work as well, consider a child's throat as a battleground for many types of bacteria vying for dominance. Repeated exposure to penicillin and amoxicillin shifts the center of microbial power, resulting in a throat full of bacteria that actually shield strep germs from the older drugs. At least four types of bacteria that Pichichero called "co-pathogens" become more dominant in the throat of someone who has received the drugs frequently. It happens in much the same way that the hardened thugs who protect a mob boss are chosen: The best henchmen are those who have survived multiple attempts on the boss's life and have become experts at snuffing out future attacks, providing the boss immunity against threats that once were formidable.
The new results are an extension of a report last year in Pediatrics by Pichichero and Casey, who is clinical associate professor of pediatrics at Golisano Children's Hospital at Strong. There they reported the waning effectiveness of penicillin and amox
Source:University of Rochester Medical Center