> if you can eat it you certainly can put it on your skin That doesn’t always follow. There are (organic, large-molecule) substances that your mucus membranes

> if you can eat it you certainly can put it on your skin That doesn’t always follow. There are (organic, large-molecule) substances that your mucus membranes will protect you from, and which you’ll then digest (denature) and greatly metabolize (non-leaky intestinal absorption routes through the liver) and so render harmless; but which would greatly harm you if left on your skin, as the substance can potentially absorb from there all the way to your bloodstream, without any digestion or a chance at first-pass metabolism. Think, for example: testosterone gel. Eating it wouldn’t result in much if any testosterone entering your bloodstream — but rubbing it on yourself sure does.