I have a bunch of supplements which I usually take with my breakfast, washing them down with a cup of black tea. I've now delayed taking my B1 until later in the day, since I learned that coffee and tea may inactivate thiamine.
"Heavy consumption of tannin-containing or food rich in caffeine, theobromine, and theophylline (such as those present in coffee, chocolate, and tea, respectively) can inactivate thiamine, thereby compromising the thiamine status (7, 14, 15)."
frontiersin.org/articles/10...
However, I don't really know what effect it will have when you are taking large doses of thiamine. Maybe the impacts are significant, or maybe they are negligible. But it's something to bear in mind, and maybe other people want to investigate further.