Excuseable reasons for failing drug test?

If a person has their children removed from them for failing a drug test and says that it was because they were taking non-prescription cold medicine, could they get their children returned to them? I am talking about if this person's children were removed from them before because of a drug addiction problem and they again have failed a drug test during a home trial period. How likely is it that the cw or even the judge will believe that they weren't using again. :thanks: