Protective Effects of Omeprazole against Low-dose Acetylsalicylic Acid/An endoscopic controlled double-blind study in healthy volunteers. In a randomized double-blind parallel study the gastroduodenal tolerability of 300 mg acetylsalicylic acid (CAS 50-78-2, ASA) daily (8 p.m.) has been evaluated in the presence of 20 mg and 40 mg omeprazole (CAS 73590-58-6) daily (8 p.m.) or placebo in 36 healthy volunteers using upper GI-endoscopy. The treatment periods lasted 14 days. Endoscopic controls were performed at entry and repeated at day 14. At entry the mean endoscopic score averaged 0.9 +/- 0.08 in the ASA/placebo (n = 12), the ASA/omeprazole 20 mg (n = 12) and the ASA/omeprazole 40 mg group (n = 12). The median values were 1.0. In the placebo experiments 300 mg ASA daily induced marked gastroduodenal lesions at day 14 (12.4 +/- 1.7). The median value was 10.0. Concomitant administration of 20 mg omeprazole daily afforded significant protection against 300 mg ASA daily on day 14 (2.9 +/- 0.9) (p < 0.0005 vs. ASA/placebo). 300 mg ASA + 40 mg omeprazole daily reduced the damaging score to 1.8 +/- 0.5 (p < 0.00002 vs. ASA/placebo). The median values in the ASA/omeprazole 20 mg and in the ASA/omeprazole 40 mg group were 1.0. The difference in the damaging score between ASA/omeprazole 20 mg and ASA/omeprazole 40 mg after 14 days did not reach statistical significance. Our data suggest that coadministration of 20 mg omeprazole daily reduces almost completely gastroduodenal lesions evoked by 300 mg ASA.