The link was suspected when neighbouring Norway and Sweden were found to have tenfold differences of surgical anaphylaxis deaths. Sweden had no products approved containing pholcodine, whereas 40% of the population in Norway had consumed the single approved pholcodine product.
The link between the two countries was suspected due to a significant difference in surgical anaphylaxis death rates. Sweden's lack of pholcodine-containing products and Norway's high consumption rate of the single approved product were factors contributing to this discrepancy.