Here's the details from the previously mentioned study on co-sleeping that shows it can be safe or safer than non-co-sleeping: "The proportion of SIDS infants found cosleeping in a bed with parents who had drunk two units or less of alcohol and taken no drugs was no different from that of the random control infants (18% v 16%). If parents who regularly smoked were further excluded, then five of the SIDS infants (6%) were found in this less risky cosleeping environment compared with nine of the random control infants (10%)."

http://www.bmj.com/content/339/bmj.b3666

TLDR: In this study, SIDS was more likely if the co-sleeper had ingested drugs/alcohol, if the sleeping occurred on a couch, or both. Sober co-sleeping on a bed was as safe or safer than not co-sleeping.