My posts included the comments "...assuming the drug websites such as Mayo Clinic's don't recommend against taking these particular meds (a given drug and a given herb) at the same time," as well as, "Usually --but not always -- herbs can be taken at the same time as prescription drugs."
Drug websites such as Mayo Clinic's routinely post lists of known interaction issues for a given drug, although they don't cover herbs as well. DrugDigest.Org has a great med interaction guide which also includes all the common herbs. Between these two (and there's a number of other good sites as well), such sites cover interactions between drugs and foods (mainly caffeine and alcohol issues), drugs and nutritional supplements (calcium, magnesium, zinc, etc.), drugs and retail med's (Pepto Bismol, aspirin, etc.), and same for the better known herbs. For Doxycycline, Mayo Clinic lists eleven compounds to avoid, including calcium supplements.
I would suggest that:
-People routinely and safely mix common herbs (except when medical guides list specific interaction issues).
-Doctors routinely and safely mix drugs and antibiotics (except when medical guides list specific interaction issues).
-In general, one can safely mix drugs and the common herbs (except when medical guides list specific interaction issues).
-Whether it's a food, nutritional supplement, antibiotic, other drug, common herb, caffeine, or alcohol, it's safe to mix them in standard quantities (except when medical guides list specific interaction issues).
As with any rule, there's certainly exceptions. And with the less common herbs -- and certainly obscure herbs -- interaction issues are less known.