Absolute contraindications in relation to potential drug interactions in outpatient prescriptions: analysis of the first five million prescriptions in 1999

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Abstract
Objectives Adverse drug interactions increase morbidity and mortality. To prevent these, situations leading to adverse prescriptions must be clarified. This study quantifies and analyses prescriptions with potential adverse drug interactions in primary health care in the north of France over a 3-month period. Methods All prescriptions administered between 1 January and 31 March 1999 were analysed to identify potential interactions amongst drugs appearing on the same prescription sheet. The regional French healthcare database was compiled to further classify contra-indications. Results There were 5,358,374 prescriptions administered to 44% of the overall population of the Nord-Pas de Calais area (1,754,372 patients/3,990,167 general population). There were 14,390 prescriptions classified as either absolute (26%) or relative contraindications (74%). Nine drug categories accounted for most of the absolute contraindications: dopaminergic antiparkinsonians, neuroleptic agents, migraine treatments (such as ergot alkaloids, sumatriptan and other triptan derivatives), prokinetic drugs (cisapride), antibacterial drugs (macrolides), antifungals (imidazoles), antiarrhythmics, betablockers and analgesics (opioids and floctafenine). In 54% of patients exposed, the incurred risk was either QT prolongation/Torsade de Pointes or antagonism of dopaminergic antiparkinson agents with dopamine receptor antagonists prescribed as antipsychotic agents. Conclusions Among a non-selected population of ambulatory outpatients, the number of quarterly prescriptions with contra-indications with potentially harmful drug interaction was 27 in 10,000 prescriptions. This would extrapolate to nearly 200,000 contra-indications on the same-prescription sheets in France in the first quarter of 1999.