Seven healthy subjects received, after an overnight fast, a single 200-mg oral dose of norfloxacin with water, whole milk, and unflavored yoghurt. Coadministration of milk or yoghurt reduced the extent of norfloxacin absorption and the mean peak concentration in plasma by approximately 50%. Taking of norfloxacin with these liquid dairy products should be avoided.Food-drug interactions have been studied extensively in recent years, but because of the complex nature of these interactions, general guidelines for clinical practice cannot be given (8,15). Food can influence drug absorption by a wide variety of mechanisms, resulting in changes in both the rate and the extent of bioavailability. The formation of poorly absorbed chelate complexes is one important mechanism of absorption interactions. A classic example is the interference of milk (calcium), iron salts, and antacids containing polyvalent cations with tetracycline absorption (7). Similarly, the absorption of fluoroquinolones is considerably impaired in the presence of cations such as calcium, iron, magnesium, zinc, and aluminum (5, 10-12).Concurrent ingestion of food with fluoroquinolones has been shown not to interfere with their absorption to a clinically significant degree (4,6,14). For example, Frost and coworkers (1) found that the bioavailability of ciprofloxacin is unaffected by a breakfast containing a high amount of calcium. However, we recently demonstrated (9) that the bioavailability of ciprofloxacin is significantly reduced when it is taken together with 300 ml of milk or yoghurt. To our knowledge, the effects of dairy products on the absorption of other fluoroquinolones have not been studied. The objective of this study was to characterize the effects of two liquid dairy products, whole milk and unflavored yoghurt, on the absorption of norfloxacin.A randomized, three-period, crossover design was used. Each treatment period was separated by a 7-day washout period. Four male and three female volunteers, aged 19 to 24 years and weighing 55 to 90 kg and considered healthy on the basis of medical history, physical examination, and routine laboratory tests, participated in the study. The volunteers were thoroughly informed in writing, and verbal consent was obtained. The study protocol was approved by the ethics committee of the Turku University Medical School.After an overnight fast, the subjects ingested 200 mg of norfloxacin (one 200-mg Lexinor tablet; Astra Pharmaceuticals, Sodertalje, Sweden) with 300 ml of water, 300 ml of whole milk (Valio, Helsinki, Finland), or 300 ml of unflavored yoghurt (Valio). The temperature of water, milk, and yoghurt was 12 to 14°C at the time of administration. The milk and yoghurt portions contained 360 and 450 mg of calcium, 9.6 and 11.4 g of protein, 11.7 and 7.5 g of fat, and 14.1 and 15 g of carbohydrates, respectively. The subjects were allowed to undertake their normal daily activities immediately after receiving the norfloxacin dose; fasting was continued for 3 h. Blood samples for the norfloxacin assay * ...