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This double-blind dose-response crossover study was designed to compare the efficacy and tolerability of sustained-release (SR) and conventional diltiazem over 4 weeks in patients with stable angina pectoris. Following a 2-week placebo run-in period, 26 patients were randomised into 3 parallel groups to receive either diltiazem SR (180, 240 or 300mg once daily) or conventional diltiazem 60mg three times daily for 2 weeks. Treatments were then crossed over for a further 2-week study period. Antianginal efficacy was evaluated using submaximal treadmill exercise testing. At baseline, all 3 treatment groups were comparable for all parameters. Treatment with both conventional and SR formulations improved exercise time and reduced intensity of ischaemia, but the difference between groups at the end of each 2-week treatment phase was not statistically significant. The results did not suggest a dose-response relationship. Adverse reactions necessitated treatment discontinuation in two patients being treated with conventional diltiazem and in one patient receiving diltiazem SR 240mg. In conclusion, this study demonstrated that diltiazem SR 180, 240 and 300mg did not differ from the conventional formulation in terms of anti-ischaemic efficacy.