Our Chronic Pain Forum sticky...I have a long post there on drugs and QT.
Including 4 lists of them from Univ. of Arizona.

Sometimes dose is important. But one thing you MUST understand is that if
you are taking two or more potential QT prolonging drugs, and are stable, if you develop dehydration, low potassium and/or low magnesium from any reason, you could tip into a QT event.

This would be poor diet, the flu, vomiting, or use of diuretics suddenly that deplete potassium and magnesium.

This is what happened with the drug cisapride (Proplusid) which when given with diuretics for people with high blood pressure, became then dangerous when magnesium levels dropped. That drug was taken off the market, because of doctors failure to heed to warning letters sent out.

Not all pain management doctors understand the risks of methadone.
So you should really have a baseline EKG done to see if you are a hidden risk to QT, before starting methadone treatment.

But the bottom line is normal people can suffer a QT event based on situations that come up with use of the various drugs and the combination of electrolyte changes.