I actually raised the question on these boards (on the Odds and Ends Board) of what prescription drugs might the murderer have been taking.

Here's one apparantly: Vicodin.

This seems less a matter of his taking it than of his deliberately "dosing up" on it:

Vicodin side-effects can include euphoria, paranoia and, in rare cases, hallucinations. It will be interesting to see if the media will continue to ask questions regarding Holmes’ possible prescription drug abuse, especially given the fact that he was a medical student specializing in neuroscience.
Colorado Mass-Murder Linked to Prescription Drug Use | Health Impact News