AbstractThe pharmacokinetics of two new sustained‐release (SR) products of diltiazem, Dilapress 120 mg tablets and Dilapress 240 mg tablets, was analysed and characterized in three different studies, in comparison to the following diltiazem SR formulations: Cardizem Retard, Cardizem SR, and Cardizem CD. Dilapress 120, designated for twice‐daily dosing, was found to be bioequivalent to Cardizem SR and to Cardizem Retard with mean (±SD) relative bioavailability values of 99 ± 27% and 113 ± 38%, respectively. Dilapress 240, designed for once‐a‐day treatment, was found to have a slower absorption rate than Cardizem SR and its extent of absorption was 56 ± 19% relative to that of Cardizem SR. However, the bioavailability of Dilapress 240 relative to that of Cardizem CD was 118±46%, indicating that the bioavailability of Cardizem CD relative to that of Cardizem SR was only 54±29%. Diltiazem is partially available due to a saturable liver first‐pass effect. A high dose of Cardizem SR may partially escape this first‐pass effect and, thus, achieve a higher extent of absorption than a slower SR product. Consequently, SR products of diltiazem designed for once‐daily treatment may not reach the saturation stage in the liver first‐pass effect process that diltiazem is susceptible to. Consequently, a twice‐daily SR product of diltiazem cannot serve as a reference for extent of absorption assessments of