Kinetics and dynamics of furosemide and slow-acting furosemide

Bioavailability and dynamics of a sustained-release preparation of furosemide (FR, 60 mg) were compared with those of a conventional tablet (F, 40 mg). The preparations were given to 12 healthy subjects in a study of crossover randomized design once daily for a week. FR absorption was substantially delayed and the uptake of furosemide was about 75% of that from F, despite the larger dose administered. F induced a brief, intense diuresis and excretion of Na+, K+, and Cl-, both during short- and long-term administration. There was no such peak after FR. Total diuretic and saluretic effects did not differ between the two preparations, despite the lower bioavailability of FR. It is suggested that this discrepancy might be caused by transient supramaximal urinary levels of furosemide after F, whereby not all of the drug can exert an effect.