As was said, several SSRI's/anti-Depressants are strong inhibitors of CYP2D6, and CYP2D6 is the enzyme responsible for converting on average 5-10% of the Codeine ingested into Morphine. Celexa, Prozac, Paxil & Wellbutrin are all anti-Depressants that could negatively impact the conversion of Codeine to Morphine. This isn't written in stone that if you are taking these anti-Depressants (or another strong inhibitor of CYP2D6) that it will block the euphoria of Codeine, or prevent the conversion to Morphine; just that it is a good possibility.
Strong inducers of CYP2D6 should be avoided as well. Glutethimide (Doriden) being the most famous example; a strong CYP2D6 inducer that when taken with Codeine ups the average percentage of Codeine to Morphine conversion from 5-10% to 30%. Taking a strong 2D6 inducer with Codeine could lead easily to an accidental fatal overdose in someone who is not tolerant or dependant.