Here is the summary:
1. Sparingly soluble drugs are transferred rapidly from the mouth into the mucous membrane. This process is driven by the lipophilic nature of the membrane. Typically 80% of the drug is transferred in the first two minutes.
2. Transfer from the mucous membrane into the bloodstream is usually slow. It is this transfer that is the rate-limiting step.
3. The mucous membrane is a storage reservoir for the drug.
4. After swallowing, or rinsing the mouth with water, the drug can diffuse back into the mouth.
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This model explains some of the observations made by physics envy and myself while quidding leaf or powder.
1. Doubling a moderate dose of leaf does not double the intensity of the trip. All it does is extend the duration of the trip. A simple analogy would be emptying a tank of water with a pump. It will take twice as long to empty a full tank, as it would to empty a half full tank.
2. My moderate trips end abruptly. Reason: There is no more salvinorin in the membrane, and the flow of salvinorin into the bloodstream has stopped. In other words, the tank is empty.
A different mechanism applies when we take a big dose. The trip is more intense, and lasts longer. I can only speculate here. Salvinorin and other insoluble material is surrounded by layers of solubilizing molecules, like phospholipids. The salvinorin particles exist as micelles. These micelles readily move into the mucous membrane.
Light dose: The micelle fuses with the cell walls of cells within the mucous membrane. Salvinorin migrates out of the micelle and into the lipophilic material in the membrane. From there, it diffuses into the bloodstream. Hydrophylic substances that carried it into the mucous membrane are either ejected back into the mouth, or metabolized in the membrane.
Heavy dose: The lipophilic material in the mucous membrane is saturated with salvinorin. The excess salvinorin passes straight into the bloodstream, either as stripped salvinorin, or some form of solubilized salvinorin. In terms of the analogy, the tank is filled to overflowing.
I have no way of proving this actually happens. I am a visual learner, and like to think in terms of pictures. I will gladly discard this view if something better comes along.