Seemingly inexplicable pricing is widespread in the U.S. market for generic antibiotics. Since 2012, list prices for tetracycline, which treats pneumonia and urinary tract infections, have soared to 170 times the old price. And some varieties of erythromycin, used for strep throat and other respiratory tract infections, have risen 1,000 percent since 2010. “It's a market failure,” said Craig Garthwaite, a health economist at Northwestern University's Kellogg School of Management.