How early can I start taking B6 and Unisom preventatively?

I'm currently 3 weeks 4 days and already feeling lightheaded and nauseous, but I suspect a lot of it could be anxiety. want to do everything I can to prevent morning sickness from becoming unmanageable. I have the activated form of B6 (P5P) and Unisom tabs on hand. Can I start taking them now as a preventative, or would taking them too early cause a tolerance buildup, making them less effective later?

Any advice or personal experiences would be really appreciated.