DRUGS!
Serously - Immodium, Motilium, Indigestion tablets, Antihistamines etc - They all add up - Buy a home made drug kit - Ie a small pouch (remember they will have no spare space), With basics for where they are going.
I have bought and received the above plus Diarylite (rehydration salts) a little piezo electic mosquito bite zapper thing (It worked on relieving the itching), tweezers, safety pins etc. Even simple things as a waterproof case with some condoms,band aids etc
Just make sure you put the instruction leaflet in the pack with the blister packs. Both for instructions on how to use and to show customs if ever they are asked what the tablets are for.