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OBJECTIVE: Oral metronidazole used in combined regimens for Helicobacter pylori eradication has been associated with an increased risk of acute pancreatitis; however, it is less clear whether a similar association exists for single-regimen metronidazole. We, therefore, examined the association of single and combined regimens of oral metronidazole with risk of acute pancreatitis.

METHODS: In this population-based case-control study, all individuals in Sweden (aged 40-84 years) hospitalized with acute pancreatitis between January 2006 and December 2008 were identified from a national hospital register (n=5,996). Controls, matched for calendar year, age, and sex, were randomly sampled from a national population register (n=60,681). Data on oral metronidazole and covariates were extracted from national health and prescription registers. Odds ratios (ORs) of acute pancreatitis, according to timing of the latest metronidazole prescription before hospitalization, were estimated using logistic regression models. Confounding by indication was examined by contrasting the main results with the association when amoxicillin was used as exposure. The robustness of results was examined by calculating incidence rate ratios using a self-controlled case series approach.

RESULTS: After adjustment for potential confounders, there was a substantially increased risk of acute pancreatitis within 30 days of oral metronidazole exposure, both for single (OR: 4.06; 95% confidence interval [CI]: 1.90-8.64) and combined (OR: 11.80; 95% CI: 6.86-20.28) regimens, compared to nonexposure. In contrast, the adjusted OR was 1.79 (95% CI: 1.25-2.54) for current use of amoxicillin compared to nonexposure. These results were supported by the self-controlled cases series analysis (incidence rate ratio: 3.30; 95% CI: 2.69-4.06, for single and combined regimens of oral metronidazole pooled). There was no strong association between oral metronidazole and acute pancreatitis more than 30 days after exposure.

CONCLUSION: There was an increased risk of acute pancreatitis within 30 days of exposure to single and combined regimens of oral metronidazole. While reverse causality and confounding by indication cannot be entirely excluded, they are unlikely to fully explain the association. These results warrant an increased awareness among physicians.