Can tramadol really induce hyponatraemia? A pharmacovigilance study

Several papers have described hyponatraemia with tramadol. However, in most reports, several confounding factors can be found. We used the WHO pharmacovigilance database (VigiBase®) to investigate if tramadol alone could be associated with hyponatraemia. All 1992-2019 ICSRs (individual case safety reports) with the preferred term (PT) "hyponatraemia" and tramadol were included. Two disproportionality analyses were performed: (1) after inclusion of all reports, and (2) after exclusion of concomitant hyponatraemic drugs. Results are expressed as reporting odds ratios (ROR; 95% CI) and information component (IC). Of 19 747 604 ICSRs, 225 575 were included. A significant association was found between tramadol use and reports of hyponatraemia (ROR = 1.49 [1.39-1.60], IC = 0.57 [IC₀₂₅ = 0.47]). After exclusion of hyponatraemic drugs, the previously found association disappeared. The study failed to find any pharmacovigilance signal of hyponatraemia with tramadol alone. We suggest that reports of hyponatraemia with tramadol can be explained principally by other underlying causes of hyponatraemia, especially other concomitant hyponatraemic drugs.