I take an im injectable medicine and forgot to swab off the top w alcohol. should i consider it contaminated or is it still ok (just forgot that once)?

No alcohol wipe: This sometimes happen. The alcohol is a routine precaution against skin infection. It is not 100% guaranteed. It is common that in home use the wipe might be forgotten. I have read a medical journal article that documented that it was safe for diabetics to inject their Insulin through their pants! you are very probably not going to have a problem, but always be on the lookout for signs of infectio.

Why take a chance?: There are articles (here's one: http://www.Ncbi.Nlm.Nih.Gov/pmc/articles/pmc272643/pdf/jcm00139-0223.Pdf) that say contamination of a multidose vial is not a significant risk. Nevertheless there is a chance you pushed some bacteria into the vial, and i know my wife (a nurse) would say throw it out and start a new one. Why take the chance? Medicine is (relatively) cheap, infections are expensive.