Swallow it.
This, if you think there's still a lot to come and you're already tripping you can spit it out if you want but swallowing the tab has more or less the same effect, you're still going to be tripping for a good while but the rest will be absorbed much slower in your stomach.
I had a trip with some friends where I realised after nearly two hours that my tab hadn't dissolved and was still in my mouth, I asked my friends about theirs and apparently they had dissolved an hour ago. After that I decided to just swallow it and then realised pretty quickly that I was tripping a forgetload harder than they were, so it's also a good idea if you're tab is still lingering around after too long.