Lithium to Lamictal

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My father is in his early 60s, he's been on some amount of lithium since his mid 20s, almost 40 years. He had a genuine manic episode back then, delusions that people in the TV were watching him, that someone was planting spiders in the bath tub. When he was around 30 a doctor tried to either lower his dose or took him off it completely, then he went on a cross-country trip for work and had an even worse manic episode. At some point in the last few years, my mom mentioned that his internist (doesn't have a psychiatrist any more) lowered his lithium, because of mad blood test results, I think liver (could be kidney).

I'm not sure how much it changed his mood, but he does seem to be off in his own world, not really engaging with the rest of us. That could just be him aging, though. Every once in awhile he says something that sounds genuinely paranoid, but it's not related to his mood or him being really wired, it's apparently just something his natural thought process leads him to.

He developed a hand tremor months ago, honestly, maybe even more than a year ago. His old internist is retiring, so he was just switched over to a younger internist in the same practice. This new internist noticed the tremor and wants to lower his lithium dose and add in Lamictal.

I have no idea what to expect from this. Has any one else done this and worked out well? Or really badly? I'm most worried that he would start to get into a more manic state, but that he wouldn't realize it, and would refuse to go back to the doctor. He is not all that self aware normally. I also see info online about bad skin reactions and that's something I seem to have a genetic predisposition towards, and he might have had happen more often than I am aware.