Do you have documentation that you were outside the US recently? Unfortunately these people look at drug tests as evidence you were under the influence at that very moment in time. My only suggestion is to pull a bank statement for the time you purchased said OTC drug. Hopefully with proof of you being outside the US and documentation of a purchase at a location with codeine sold OTC they will be lenient.
its a gamble, self reporting makes it easier for everyone because it keeps a mark off your license and speeds the whole damn process up, but you have somewhat of a defense here. These people are not reasonable though