Methadone produces no serious side effects, although some patients experience minor symptoms such as constipation, water retention, drowsiness, skin rash, excessive sweating,
and changes in libido. Once
methadone dosage is adjusted
and stabilized or tolerance increases, these symptoms usually subside. From what I was able to read, there are no real side effects of Methadone that would cause the issues you are experiencing.
Hope this helps.