They can both definitely be used at the same time. Retin-a basically unplugs your pores to stop the bacteria from building up in them and erythromycin is an antibiotic that kills the bacteria, so they complement each other quite well. Infact there is a med out there that has both in it - aknemycin plus - i used it a few months ago and it worked great, lots of peeling and took about 3 months to clear completely though. The combination you're on seems like a good one to me so hang in there!