This is essentially an extension of traditional, hands-on epidemiology, which assembles all patients that are prescribed a drug into a cohort which is then followed. In the UK, for example, through the Drug Safety Research Trust, all or a sample of this cohort is assembled from the records of the prescription pricing authority, generally within the first year or two of initial marketing of the product. Each patient can be followed-up with a confidential enquiry for serious adverse events using a form that, in the UK, is popularly called by its appearance: the 'Green card' (this term has an entirely different meaning in the USA, and, curiously, describes a pink document!). It is a classic example of an observational as opposed to an experimental method, in which all uses and all outcomes (events) are observed, generally without a simultaneously collected comparison population. Thus, data stemming from these sorts of activities are fraught with analytical and methodological traps. However, PEM is a good method for generating hypotheses for further testing, usually after reconciliation with the known pharmacology of the drug of interest, other drugs in the same class, and the natural history of the disease and kindred disorders. These, of course, shed further light on these data, and may be gathered, e.g. from the spontaneous reports system. Indeed, sometimes this is also the first evidence of an unsuspected drug intoler-ability, perhaps in a previously unsuspected subset of the treated patient population. The ability in certain European areas and New Zealand to aggregate prescriptions from entire countries or regions, often as part of the reimbursement system, is obviously strategic to this approach.
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