The aim of this study was to create a standard set of essential drug information items as a tool to assess the completeness of any type of drug information source, regardless of its length, using a Delphi consensus panel of European health care professionals. A compilation of drug-related information items was performed by searching several resources for health care professionals and a final list of 162 items was obtained. Fifty-seven experts in drug information from 23 different European countries were invited to participate in a three-round Delphi technique to obtain consensus on items considered essential and non-essential content of information. Consensus for the first, second, and third rounds was defined as ≥90%, ≥80%, and ≥75% agreement, respectively. Of the 57 experts invited, 32 completed the first round, 27 the second, and 29 the third. Consensus was achieved for 28.3% of the items in the first round, 49.3% in the second, and 58.3% in the third. The final cumulative consensus was 67.7% (n = 126) for items considered essential and 16.1% (n = 30) for items considered non-essential. The final tool obtained to assess the completeness of drug information sources was composed by 126 essential items grouped into 11 sections. This tool allows for the comparison of different information sources for the same medicine and the information content for different medicines in the same source.