Charcoal works BUT it interferes with absorption of everything else you put in your digestive system - your meds, as you say, and the nutrients from your food. It buggers up your gut flora, as well. It's great if you've got something important to attend and would rather not be emitting gases everywhere, but unwise to take it regularly.
Onions are a sulphur-rich food, like brassicas (cabbage, broccoli, etc), beans, eggs and a few other things. Sulphur makes some people very gassy, and also makes farts very smelly! Trouble is, our bodies need sulphur and the very foods providing it are also full of other goodness.
Some things that might help (no guarantees):
. Don't eat onions or beans
. Do more flexibility exercises
. Try taking probiotics
. Don't drink beer, cider or wine
. Definitely don't drink fizzy drinks
. Try to reduce sugar
. Reduce dietary fibre
. Try yoghurt before meals (old wives' tale)
Or ... just blast away, saving the charcoal for critical moments!