About 50 to 60 percent of women are diagnosed with a urinary tract infection (UTI) at least once during their lifetime. Many experience multiple recurrences. Cranberry juice, a popular home remedy for UTI, is often taken along with low-dose antibiotics as a preventive measure. Because little is known about the potential of cranberry juice to interact with drugs, NCCIH-funded researchers at the University of Washington studied cranberry's effects on two antibiotics frequently prescribed for UTI: amoxicillin and cefaclor.
Two parallel studies were conducted, one for amoxicillin and one for cefaclor. In each study, 18 healthy women took a single oral dose of the antibiotic, with either water or cranberry juice cocktail. The procedure was repeated after a week, in a “crossover” design that allowed researchers to evaluate cranberry juice in each participant. The drugs were tested at commonly prescribed dosages: 500 mg for cefaclor in one study; and 500 mg, as well as 2 g for amoxicillin in the other study. Participants drank 8 ounces of juice or water with amoxicillin, 12 ounces with cefaclor; those who drank juice also drank it twice daily for 2 days before the treatment, to approximate real-life consumption. Researchers took blood and urine samples at 2-hour intervals for 6 to 8 hours after treatment, and then tested drug concentrations at each point.
The data showed that cranberry juice did not significantly affect either antibiotic's oral absorption or renal clearance (i.e., how completely the body processed the drugs in the intestine and kidneys). Absorption took somewhat longer with cranberry juice, but the delay was small, and the total amount of antibiotic absorbed was not affected.
Based on these results, the researchers concluded that cranberry juice cocktail, consumed in usual quantities, is unlikely to change the effects of these two antibiotics on UTIs. They noted that the same may or may not be true of other antibiotics, or when people who take antibiotics also drink a large quantity of concentrated cranberry juice.
Li M, Andrew MA, Wang J, et al. Effects of cranberry juice on pharmacokinetics of beta-lactam antibiotics following oral administration. Antimicrobial Agents and Chemotherapy.;53(7):2725–32.2009