What would you do if a patient suddenly stopped phenytoin treatment and they were also on Methadone? Would you alter the dose as a standard response, would you see how things went and review clinically? Or is this a non-issue as the liver inducing effects of phenytoin will wear off slow enough to allow tolerance to develop to the relative increase in methadone dose?

I'd warn the patient of the effects and give advice to take less if needed or maybe to split the dose for a short while. Review more regularly.