At one point I was feeling a bit mentally groggy, decided to start taking a B1 supplement. 250mg thiamine mononitrate. It worked well, more mental clarity, but after 2-3 days, must've built up, started having anxiety, near attack levels.
It was not similar to the stimulation from caffeine/etc which do not cause me any anxiety in my regular daily dose of a couple cups of coffee, and tea in summer. There was no crash from the B1, quite the opposite, harder to sleep.
Stopped taking that B1, switched to a B-100 multi with 100mg thiamine HCL, every other day. I've been contemplating further dialing that back, seems like the lower dose still interferes with getting to sleep. I'm not one to take yet another suppliment to try to counter the first one, so not going with melatonin or other options.
It's also more of a seasonal thing, I get a lot more exercise in spring through fall and sleep better. It may also be the shorter winter days and colder hue of LED lighting, including a larger computer monitor, are messing with my sleep patterns, that it's just the cumulative effect but there's no mistaking the sudden change in anxiety I had when I was taking the 250mg thiamine mononitrate daily. It wasn't social/situational anxiety... no triggers involved.
Megadoses of niacin feel to me more like a flush combined with a mild sunburn, affects me only for a half hour or less but only certain forms or doses of niacin. The 100mg niacinamide in my B-100 complex, doesn't cause that at all.