We collected two drug databases of adverse drug-drug interactions: the database available through the web site Drugs.com and the DrugBank database. For the Drugs.com database, we restrict our analysis to major adverse interactions between generic drugs; for the DrugBank, we consider all interactions.

There are two snapshots of each of the databases. For the Drugs.com database, we collected the first snapshot in May 10, 2010, and the second one in February 22, 2012. A total of 1,518 drugs are listed in both snapshots. There are 32,074 drug interactions present in both instances of the network. For the DrugBank dataset, the first snapshot corresponds to January 2009, and the second to April 2012. A total of 1,012 drugs are listed in both snapshots; there are 9,113 drug interactions present in both instances of the network.

For the Drugs.com dataset, we collected all drug interactions that were listed in the website, starting from a small set of highly connected seed drugs. Drugs that are not connected, directly or indirectly, to the seed drugs are not included in our analysis. We limited our searches to “generic drugs” (which include common combinations of generic drugs such as acetaminophen/hydrocodone) and to “major interactions.” We consider two snapshots of the database from May 10, 2010, and February 22, 2012.

Finally, for the DrugBank, we downloaded two snapshots of the complete database, corresponding to January 2009 and April 2012, from http://www.drugbank.ca/downloads.