People with drug problems obviously weren't teetotalers during their time of abuse, so you could see what their pattern was with regard to drinking. If their issue had always been with drugs or a particular drug and not alcohol, there would be no reason to think that alcohol would trigger them to consume their former drug of choice or that they would be unable to handle it. Of course, if they or others around them began to notice that they could not control their drinking, they should stop ASAP.