@CCC Could you say more? What changed when you took the sublingual FMN?
The most obvious effect of FMN in terms of vitamin D was that sitting in the sun suddenly did something positive. Nothing else changed, and days with FMN were different from days not with FMN (inconsistent compliance in the first week or so).
Then, a biochemist told us that B2 generally has a role in Vitamin D metabolism (converting to a usable form), so that was probably the reason. The biochemist didn't think FMN offered any advantage over the cheaper B2, but our experience says otherwise. It could be the sublingual effect for all I know.
Another obvious and immediate effect is that FMN takes away some of the inflammation symptoms of methylation that methylfolate doesn't quite nail.
More generally, the effect of FMN was to turn the lights up. It took a month of consistent, low dose FMN for this (18 mg sublingual at the rate of 1/4 each morning and night, with the occasional top up if the body required it). We've been told that it takes a month for the B2-dependent enzymes to replenish, so that's probably why it was a month.