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.
A link between the two countries was suspected due to a significant difference in surgical anaphylaxis death rates. Sweden, which did not have any pholcodine-containing products approved, had ten times fewer deaths than Norway, where 40% of the population had consumed the sole approved pholcodine product.