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Aims: To evaluate absorption of protein-bound and unbound cyanocobalamin before and during treatment with omeprazole, and cobalamin levels in patients on long-term treatment with omeprazole.

Methods: In eight former duodenal ulcer patients absorption of unbound and protein-bound cobalamin was determined by measuring 24-h urinary excretion of unbound 58Co-cyanocobalamin or protein-bound 57Co-cyanocobalamin during a modified Schilling test. Tests were performed before and during treatment with 20 mg and 40 mg omeprazole daily for 9 days.

Serum cobalamin levels were assessed in 25 patients with gastro-oesophageal reflux disease (GERD) before and during long-term maintenance therapy with omeprazole. Mean treatment duration was 56 months (range 36–81 months).

Results: Urinary excretion of unbound cobalamin was unchanged with both dosages of omeprazole. Excretion of 57Co-cyanocobalamin, however, decreased significantly during treatment with both 20 mg omeprazole (mean ±S.E.M.: 1.31±0.20 vs. 0.54±0.17%; P<0.02) and 40 mg omeprazole (1.25±0.26 vs. 0.29±0.06%; P<0.02).

Mean serum cobalamin levels (±S.E.M.) before and during therapy with omeprazole in GERD patients were 298±27 and 261±16 pg/mL (normal range 180–900 pg/mL), respectively (P=N.S.).

Conclusions: Absorption of protein-bound, but not unbound, cyanocobalamin is decreased when measured by a modified Schilling test during treatment with omeprazole. However, no change in serum cobalamin levels was observed in patients with GERD after treatment with omeprazole for up to 7 years.