ObjectivesTo evaluate the degree of compliance with the current guidelines regarding venous thromboembolism (VTE) prophylaxis in medical patients during admission and to identify risk factors linked to complications of VTE prophylaxis.DesignProspective cohort study.SettingThe Internal Medicine Department of the University Hospital of Santiago de Compostela (tertiary referral hospital).ParticipantsA total of 396 hospitalised, elderly patients who did not undergo surgery and had no active or previous oral anticoagulation or low molecular weight heparin (LMWH) treatment (during the previous year) and who received VTE prophylaxis during admission.Primary and secondary outcome measuresThe degree of compliance with the current guidelines was estimated by calculating PADOVA and IMPROVE indexes in all cases. We analysed the development of the following complications: major and minor bleeding, major and minor haematoma and decrease of platelet count.ResultsWe found that VTE prophylaxis was correctly indicated in 88.4% of patients. We found two (0.5%) cases with major bleeding, 17 (4.3%) with minor bleeding, 30 (7.6%) with decreased platelet count, 29 (7.3%) with major haematoma and 82 (20.7%) with minor haematoma. After multivariate logistic regression analysis, the presence of major haematomas was linked to obesity (OR 4.1; 95% CI 1.8 to 9.2, p=0.001), concomitant antiplatelet treatment (OR 2.7; 95% CI 1.1 to 6.5, p=0.03) and enoxaparin use (OR 3.5; 95% CI 1.1 to 10.9, p=0.029), and the presence of minor haematomas was associated with PADOVA index <4 points (OR 3.1; 95% CI 1.5 to 6.4, p=0.003) and diabetes mellitus (OR 2; 95% CI 1.1 to 3.7, p=0.031).ConclusionsComplications during VTE prophylaxis in elderly hospitalised medical patients are frequent even with correct application of current guidelines. The main factors linked to haematomas were obesity and concomitant antiplatelet treatment, the presence of which should lead physicians to exercise extreme caution. The use of tinzaparin for VTE prophylaxis in these patients could have a better safety profile.