I'm now a bit embarassed of my snark, too, though I like to use the phrase "literal Nuclear Bomb" whenever possible.
Is there any reason for something applied topically that is an "alternative" medicine/cure/thing, that spot-testing first wouldn't be sufficient? That's what recommended for other stuff like BP, which many people also have negative reactions to.
Senecando,
Spot-treating can be helpful, but it doesn't pick up everything. Something like benzoyl peroxide is an irritant, so most people are going to have a mild reaction to it when they use it. This shows up in the first few days, gets worse the more you use it, and goes away quickly when you stop using it. There are allergic reactions to BP but they are much rarer and more serious.
Allergic reactions are where spot treatment may or may not help. The problem is that an allergic reaction requires an initial period of sensitization and then a reexposure that sets off the allergic reaction. An over the counter antibiotic like bacitracin is a good example. If you are destined to become allergic to bacitracin, you may show no reaction whatsoever the first time you use it. Your skin is in the process of presenting it to the immune system, which "remembers" it for the next time it is exposed to it. The next time you use it, and all other times after that, you will get a poison-ivy like reaction.
Another issue is that different parts of the skin are more or less sensitive. Something that causes a reaction on your eyelids (thin sensitive skin) may not cause a reaction on your hands (thicker skin). When I tell people to test a medication, I usually recommend they put it on the soft skin at the crook of the elbow 2-3x/day for a few days.
The last issue is that no one should ever spot test a medication that is not meant to be a leave-on product. For example, a medicated shampoo. This is not meant to be left on the skin and will almost always cause a reaction, sometimes severe.
