This is the first epidemiological study showing that: i) prior Anisakis infections causing IgE sensitization are an independent risk factor for UGIB (with an almost twofold increase in the risk) and ii) that this effect is modified by NSAID consumption, to the extent that the risk of UGIB can increase by more than 14 times through a synergic effect between Anisakis and NSAIDs, showing in turn that the joint effect of the two risk factors is 3 times higher than the sum of their individual effects. This evidence concerns the gene IGHE and anisakiasis.