El número de pacientes necesarios a tratar para producir daño. Medicamentos que salen del mercado. Europa #patientsafety *The number needed to treat to be harmed: medicines withdrawn from the European market. #patientsafety* http://www.tandfonline.com/doi/abs/10.1080/14740338.2016.1217989 *Objective: To explore the usefulness of number needed to treat to be harmed (NNTH), in benefit-risk assessments, by studying the agreement between NNTH values and withdrawals of medicines from European market due to safety reasons.* *Methods: Medicines with data from longitudinal studies were included. Studies were identified from European Medicines Agency’s Reports. Meta-analyses were performed to pool odds ratios (OR) with 95% confidence-intervals (CI). Published control event rates were applied to ORs to calculate NNTHs (95%CI) for selected adverse events.* *Results: NNTH (95%CI) decreased from pre- to post-marketing for the eight medicines included: peripheral neuropathy (∞ vs. 12[non-significant; NS] with almitrine; heart valve disease with benfluorex (∞ vs. NNTH ranging from 7[4–13] to 7[5–9]); myopathy (−4096[NS] vs. 797[421–1690]), new-onset diabetes (113[NS] vs. 390[425–778]), bleeding (∞ vs. 517[317–1153]), and infection (∞ vs. 253[164–463]) with niacin-laropiprant; psychiatric disorders (12[7–34] vs. 9[5–24]) with rimonabant; myocardial infarction (MI) [−1305 vs. 270[89–4362]) with rofecoxib; MI (−510 vs. NNTH ranging from 152[55–4003] to 568[344–1350]) with rosiglitazone; cardiovascular events (∞ vs. 245[129–1318]) with sibutramine; and liver injury (∞ vs. 5957[NS]) with ximelagatran.* *Conclusion: NNTH have potential of use as a supportive tool in benefit-risk re-evaluations of medicines and may help regulators to making decisions on drug safety.* -un saludo juan gérvas