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.
"...if you are taking doxycycline as I just stated that I am (together with iodoquinol) for the treatment of dientamoeba fragilis you must not take it with zinc."
I could not confirm that. The standard medical guides for both Doxycycline and Iodoquinol (links below) do not mention zinc interaction issues, although those with "zinc deficiency" may have issues with Iodoquinol. (Mineral supplement interactions are certainly well-covered by Mayo and others since millions of folks take supplements such as Centrum. Examples include no iron with Ciprofloxacin, no calcium with Doxycycline, etc.).
Going back to the original posts (taking something called Citramesia along with prescription drugs for the DF parasite), no interaction issues were mentioned here between any combination of Doxycycline, Iodoquinol, zinc, and wormwood (grapefruit seed extract was not listed):