Remember all that talk about protonated amines yesterday? Here’s another place where it matters.
You’ll note that I’ve drawn arecoline and nicotine with positive charges. As I mention in this old entry about nicotine, this is the physiologically relevant form (at your cells’ pH, this is what you find).
The trouble with this, though, is that charged molecules don’t cross cell membranes. An oral, buccal, or sublingual drug can be made quite a bit more effective by converting it from the charged form to the freebase. For example, here is nicotine freebase:
This is what’s going on when you hear about cigarette and chewing tobacco manufacturers adding ammonia to their product – the ammonia is a pretty good base, and makes more freebase drug available to cross the lungs or buccal pouch.
“Crack,” or “freebase,” is simply deprotonated cocaine:
For this very reason, indigenous users of coca treat the leaves (which are chewed and held against the buccal mucosa) with lime (calcium hydroxide, among other things). This makes much more cocaine freebase available, which allows for more effective delivery of the active drug – although not at nearly as high a rate as you get from taking in the pure substance, naturally.