Written evidence from George Washington shows that he smoked it medicinally (he wrote that it was "too late" to separate the male from the female hemp plant; there is no reason to do so unless you're interested in smoking it), so it's not wholly unbelievable that Jefferson might have indulged too. He must have known about its effects if Washington did, and back then nobody thought smoking was unhealthy, so I see no reason why he wouldn't.