I read threads that niacin, the FLUSHING kind, will slow methylation, according to some people. I've also seen threads that niacinamide, which is a non-flushing version, may slow methylation, according to other people.
I think I saw more threads on the flushing kind than the non-flushing kind, for slowing down methylation, but I've seen both written about as such on Phoenix Rising.
The flushing kind will say "niacin" on the bottle, not niacinamide, and will NOT have the words "no flush".