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Many medications are intended only for the brain, but are taken orally. Hence <10% finds its intended target, while the remaining >90% is at best wasted, and at worst causes unwanted side-effects.

Valproate is a classic example; it is medicated as an antiepileptic, but only a small fraction crosses the brain blood barrier to make it into the brain. Because of this, a large dose is required, and unwanted side-effects are common.

However, certain (small molecule?) drugs (e.g. Ibuprofen) can be applied transdermally.

For a drug intended for brain tissue that allows transdermal application, is there any sense in applying on the (preferably shaved) scalp?

Is this a known mechanism of administration? If so, does it have a name?

Is there any way of correctly scaling the dosage? For example, if experiment shows that 50% of substance X taken transdermally reaches the bloodstream, if it is administered to the scalp is there any way of figuring out how much ends up in the brain and how much ends up in the rest of the body? Since this is a tricky and obscure question, may I also ask this: how to go about finding an answer to this? Where else can I ask?

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    $\begingroup$ If you wanted to deliver a drug directly to the brain, it might be doable with microdialysis, where a tube is inserted into the brain and chemicals can be added or removed for analysis. It's done fairly often for research in mice, but I've never heard of it being used for drug delivery. Here's a review article: ncbi.nlm.nih.gov/pubmed/19340812 $\endgroup$ – user137 Aug 23 '14 at 20:11
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The short answer is no, it will take forever for a drug to pass through skull bones.

The usual reason to apply drugs on skin is if you want them to act right there, on the skin. Acne creams fit here. You need less drugs, you get where it is needed sooner, and you have fewer off-target effects, when compared to taking them by mouth. The brain is not "right there", so this scenario does not apply.

The second reason is convenience. Some drugs cannot by taken by mouth, because they would be degraded by the stomach or guts. In most cases, the drug has to be given through painful, expensive injections. But if such a drug does pass through skin (usually when it is a fat-loving small molecule), you may get away with applying it on skin. Testosterone is one such drug. However, it usually doesn't matter whether the skin you are applying on is close of far from the target organ. The drug will be taken from the skin by blood, get to the heart through veins, and then spread to the whole body, eventually reaching the target. There is no reason to shave your head, since it will go through the same detour before reaching the desired destination.

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