I am well aware of methadone. More than you will ever know.
Using it comes with risks, and being informed of those, can make the treatment safer. While methadone is less expensive it is also trickier to use, and people vary genetically in how they metabolize it. It can build up in the body, as the pain relief wears off. In that state it is still there, affecting other systems because the pain receptors remove it first. So when doses increase, those other effects increase as well. Combined with other drugs, or with low kidney functions, it may build up until a sudden cardiac event or respiratory depression occurs.

Please visit the links I gave you. There are some things you can do on your own, but then others you cannot. Have you told your symptoms to your pain management doctor? I hope you have.