I agree with Jeremy, 100%. Much depends on how each herb/ ingredient work. For example, because of bentonite's & activated charcoal's action of drawing, it's generally advised not to take it, internally within an hour (before or after) taking other supplements or medications - particularly in cases of anemia, where someone is taking iron, because heavy metals are one of the key targets of activated charcoal and bentonite clay. I pick these two for my example, because they are the primary drawing ingredients I personally use.
In my drawing salve, I use very few herbs - and, I choose them very carefully, for this very reason. The primary active ingredients, beyond the beeswax, carrier oils, a.c. & bentonite in my salve, are double infusions of calendula & plantain, and a strong portion of frankincense. These three seem to (at least in my
experience) work well, in conjunction with the drawing ingredients. My guess is because they get to work quickly and because they marry well, in the jar, long before they are ever applied to the wound, they work together, rather than at odds.
I've been making and using my drawing salve on myself and others, for quite a few years - probably at least 9 or 10. Invariably, after 10 - 20 minutes or so, there's a significant decrease in pain, and within 12 - 24hrs of application (often as little as 2), there's a noticeable drawing effect, as well as a noticeable healing effect. My husband has been prone to sebaceous cysts, his whole life, and they'd often take a month or two to heal, occasionally even needing to be lanced. When he finally agreed to try the drawing salve, his healing time was immediately decreased to less than a week, and if caught quickly
enough, a mere day or two - and he hasn't needed any to be lanced, since.