If you use methadone to treat opiate dependence , it is best to come off opiates first and titrate them to a methadone dose,
methadone is a potent opiate
mixing it
with other opiate puts one at high risk of respiratory depression, it also a good practice to reduce or to stop other drugs that has CNS depressant effect , e.g benzodiazepines, alcohol ( yes alcohol) , pregabalins and also to be cautious of those that causes sedative effects like most antipsychotics, sleeping tabs e.