Drug causation assessment is a systematic process, designed to estimate the probability of a drug’s implication in the onset of an adverse event. Postmarketing surveillance is very useful for geriatric medicine, because it gives a better idea of a drug’s efficiency and safety in special populations, such as older adults. Although the probabilistic reasoning used in drug causation assessment often leaves some degree of uncertainty, especially in a geriatric milieu where polymedication and multimorbidity are frequent, it is the international pooling of many individual observations that allows one to distinguish signals from noise. These difficulties are discussed on the occasion of a geriatric patient with a probable hypersensitivity vasculitis and glomerular involvement related to lansoprazole treatment. Our case’s interest stems from three elements: (1) amongst the proton pump inhibitors (PPI), lansoprazole is rarely reported of having an immunoallergic effect, (2) our data yield evidence of a probable glomerular involvement whereas PPIs have been mainly incriminated of causing acute interstitial nephritis and (3) we assume that the hypersensitivity reaction to lansoprazole was maintained by subsequent administration of another PPI, omeprazole, speaking in favour of a class effect. However, our case report presents several typical confounding factors often encountered in geriatric postmarketing drug surveillance. We discuss how such problems are taken into account in the WHO pharmacovigilance methodology.