If somebody is on supervised methadone is she still taking heroin at the same time?

No: If they are in a supervised program, there should be random urine drug screens to rule out use of illicit drugs, and to be certain they are taking and not selling the methadone.

Maybe: Methadone reduces craving; it does not prevent using other opiates.

Yes but...: Nothing stops them from ongoing use of heroine. However i doubt many facilities would continue to do so.

Possible: An addict can take whatever he or she wants. On methadone, it simply takes more heroin for effect, which adds unpredictability and risk of overdose. The natural progression of the disease is intermittent relapse. Pts are monitored closely, including urine toxicology. The goal is titrating Methadone to decrease/stop attempts at using heroin.