When I worked in a forensic unit I had plenty of treatment-resistant cases. I never saw any studies backing this but from seeing several different forensic units the accepted notion is violent mentally ill people tend to be the more treatment resistant ones.
I try to avoid polypharm whenever possible but I had cases where nothing, really nothing worked except for serious polypharmacy. E.g. high dose lithium, valproic acid, benztropine but two antipsychotics.
One thing I found very bothersome was on occasion I had patients treatment resistant even to high dosages of clozapine-and there is no good data that anything can break through this other than amisulparide mixed with clozapine but amisulparide isn't available in America.
A few times we brought cases like this for the best minds in psychiatry to tackle. Henry Nasrallah recommended reserpine. This was on a specific patient who was treatment resistant, had schizoaffective bipolar type and was known to sexually assault nurses literally at least a few times a week. When nurses were assigned to him they'd call in sick the same day. He could not be on lithium, Depakote or clozapine anymore because some idiot who had him before put him on it those meds (and they were the only ones known to work) but didn't have him do labs. I'm serious and it made the guy's ANC too low (permanently), his kidneys and liver too messed up to continue their use. (How this happened I don't know with the clozapine cause usually the pharmacist will only allow this if they see the labs themselves).
So I was going to do the reserpine and the CCO of the hospital blocked me. Told me I was being too dangerous. Now mind you this is after I already consulted with Nasrallah, had it go through with the hospital's top pharmacist, and 2 grad students lit-searched the heck out of it (one of them saying this was one of the only reasons to justify any use of reserpine-for odd cases like this) and I lit-searched the heck out of it. I also had 3 family meetings with his family telling them the risks cause this guy had been dangerous for few years and was branded with possibly no hope of ever being discharged.
So it was a no-go. The guy just kept being psychotic and manic assaulting people every few days. As bad as it was this is what long-term units are for-for patients that are not expected to get better quickly if at all.
Now mind you, keep this story wrapped up in the genie's bottle cause in usual psychiatry polypharm of these levels aren't needed. This is forensic-long-term-patients cut off other people's heads type units. Polypharm should rarely be done in usual outpatient practice where patient tells you they are feeling slightly depressed and their main complaint is they hate their boss.