The absorption of piretanide from the gastro-intestinal tract is site-dependent.

The absorption of piretanide was investigated after placing the drug in the stomach, duodenum and ascending colon under visual control. The relative amounts absorbed and the rates of absorption were estimated from the area under the curve and the total mean time, respectively. Similar amounts of piretanide were absorbed and at almost the same rate after placement in the stomach and duodenum; the area under the curve for the stomach was 250 ng h/ml and for the duodenum 243 ng h/ml, the mean absorption times being 1.97 h and 1.51 h respectively. A marked difference was observed in the rate of from the ascending colon; the area amounted to 66 ng h/ml and the mean time to 3.99 h. Although area values for the colon were significantly different from those observed with the stomach and duodenum, it must be emphasised that the amount absorbed depends on the time the drug is in contact with the absorbing surface. There is discussion of whether the differences in absorption between the duodenum and colon can be explained by the physico-chemical properties of the drug alone, or whether the results reflect a saturable transport mechanism.