As I understand it, the information you've been given is incorrect.
Clay (and charcoal) draw chemical toxins & metals to themselves. There is some argument/debate about charcoal 'adsorbing nutrition', but I've never heard ANYthing about a clay poultice or bath pulling a natural substance from the digestive tract.
Of course, clay/charcoal shouldn't be taken internally close to when one ingests pharmaceuticals (because the clay/charcoal know them to be poison, so they adsorb a percentage of them).
I believe (unless you find hard/solid evidence to the contrary) that you can dismiss that warning. And if you DO find information to the contrary, please post it so we can look it over and discuss it.