If you have your heart and lung function monitored regularly. will methadone affect you in the long term and i'm negative matter?

It may: Methadone is a full mu receptor opioid agonist which means the body will develop a tolerance to it and a higher dose will be needed to obtain the same desired effect. Methadone at high doses can cause cardiac arrest. Drug-drug interaction should be assessed for as well. The choice really depends on what it's being used for or is it even necessary.