Drug utilization studies can be useful to identify patterns of drug use in different populations, whether adult or children. One aspect is the analysis of drug use in conditions other than those authorized in the technical specifications or labelling. This thesis is an analysis of various aspects of off-label drug use after the publication of a Royal Decree 1015/2009 of 19 June regulating the availability of medicines in various special situations (including off-label drug use) and after the establishment of a rule of the Catalan Institute of Health (ICS) with standardized work procedures to review, evaluate and register the use of drugs in special situations. The publication Medicina Clínica 2014;143:327-9 describes the drugs used in conditions not approved in two tertiary hospitals during the three years following the publication of Royal Decree. There were 671 applications for 117 single drugs or combinations to use in these conditions. The main groups were antineoplastic, immunosuppressive and muscle relaxants, and the most involved drugs were rituximab (170 applications), botulinum toxin (81), omalizumab (39) and anakinra (21). The main diseases were neoplastic diseases, those affecting nervous system and sense organs and digestive disorders. The publication European Journal of Clinical Pharmacology 2014;70:1385-93 describes the analysis of indications for use, clinical outcomes (clinical response and tolerability of the medication) and cost of off-label drug use in five hospitals of the ICS during one year. A total of 226 patients were included. The median [IQR] age of patients was 46 (33-62) years; 59% were women. Patients had received a median of three previous treatments, and a lack of response (or suboptimal) was the main reason for off-label use (72. 1%). A total of 232 off-label medicines were administered for 102 different indications. The most frequent medicines were rituximab (49; 21. 1%), botulinum toxin (25; 10. 7%) and omalizumab (14; 6. 0%). In 117 (51. 8%) cases the level of clinical evidence for their use was low. A partial clinical response was observed in 82 patients (36. 3%), complete response in 71 (31. 4%) and stabilization in 11 (4. 9%). A total of 58 (26. 5%) patients had adverse effects, which in 11 (4. 9%) were severe. The median (IQR) cost per patient was € 2,943. 07 (541. 9 – 5,872. 54). The publication European Journal of Clinical Pharmacology 2013;69:1689-99 describes the indications for use, the available scientific evidence and clinical results in the short and long term, and the cost of off-label use of rituximab. A total of 101 cases of off-label rituximab use were analyzed (median [IQR] age 53 years [37. 5-68. 0]; 55. 4% women). The requested indications were mainly haematological diseases (46%), systemic connective tissue disorders (27%) and kidney diseases (20%). Available evidence in these indications were mainly individual cohort studies (53. 5% of cases), and case series (25. 7%). Short-term outcome (median 3 months [IQR 2-4]) was a complete response in 38% of cases and partial response in 32. 6%. The highest short-term responses were observed for systemic lupus erythematosus and membranous glomerulonephritis, and the lowest for neuromyelitis optica and idiopathic thrombocytopenic purpura. Some response was maintained in long-term follow-up (median 23 months [IQR 12-30]) in 69. 2% of patients with short-term response. Median cost per patient was € 5,187. 5 (IQR 5,187. 5-7,781. 3).