Sedation is a commonly side effect adjusting
methadone. It usually means they are ramping it up too quickly. Perhaps they need to back off 5-15mg for a week on it, let her body catch up and then try increasing it more slowly. Or add some small doses of
amphetamines to the mix to combat sedation. If it's still intolerable, they need to try something else. It's not unusual for there to be a prolonged adjustment period (2 months+) getting used to it. Start low, go slow <--. The falling asleep on the potty/sitting down side effect is not uncommon during the adjustment period, and sometimes it persist after. It's going to be up to her what is worth it, there are alternatives. A lot of people don't like methadone because it doesn't deliver as much euphoria as other opioids, for some people that artificial lift is important and depression does set in. It sounds to me like a strong case of the blues is at work here as well, so you have some reasons to be concerned. She may do better on another opioid for pain relief, or she may just need to give it more time to adjust - or something to battle sedation. -- But she definitely needs to get the depression in check, that can be just as crippling as the pain so keep an eye on her if she has truly lost interest in everything.
It's not just workers compensation that likes methadone. It's widely used in cancer treatment, neuropathic pain, pain management centers, the VA, phantom limb pain... Cost is a factor in that, 100 80mg
oxycontins a month is over $700 even for the generic, an equivalent amount of methadone can be had for $50 or less. IV delivery systems in the home can run a couple hundred dollars a day, and it does compete directly against that. Cost is not the only factor though - for a lot of people it provides better relief. It works as an Mu agonist, NMDA antagonist, and partially inhibits the reuptake of serotonin and norepinephrine. All of those functions are important in chronic pain control. It also weakly binds to Delta & Kappa receptors.
Are you numb yet? No. AARRGGGGGHH! I told you I wasn't numb! Sorry, I didn't believe you.