you cant mix it! literally but you can take the benadryl first and wait 30 minutes for the effectthen take the mebendazole... by doing this your making the worms sedate making them less active so...
You shouldn't have any negative side-effects. The one thing to remember with any anti-biotic, such as doxy, is that it will speed up your metabolic rate of consumption. (that's why you are not...