I had that problem for a while.
I got onto Travacalm tablets. Usually one before I leave for the boat ramp, but on a full stomach. On an empty stomach they really seem to make me drousy, but if I eat something before hand or not long after I take the tablet, the drousiness seems to be almost non-existent. The "Travacalm Original" tablets have a bit of caffeine in them to counteract the drousiness effect. There is another box of Travacalm (without the "Original" name in it) that does not have the caffeine.
Havent had a problem each time I have taken them, although I wouldn't be driving alone. Once or twice that have made me really groggy for about 15-20 mins, then it went away and all fine... but eating something seems to negate the grogginess effect. Probably get less of a hit at once through slower absorption if you have something else in your guts too