Transdermal drug delivery uses the skin as a route of systemic and local drug absorption. Compounds penetrate the stratum corneum (the outermost skin layer) and diffuse into the dermis, where they are absorbed by local blood vessels and lymphatic tissue.

For magnesium, the transdermal route is particularly valuable because oral supplementation is limited by intestinal absorption capacity. High oral doses trigger an osmotic laxative effect before therapeutic magnesium levels are achieved. Soaking in magnesium sulfate solution allows passive diffusion across a large skin surface area, achieving meaningful tissue uptake without GI side effects.

For CBD applied topically, the skin's high density of CB1 and CB2 receptors in the dermis means localized effects can be achieved at concentrations far lower than those required for systemic effects. Topical CBD reaches local tissue — muscle, joint, connective tissue — without significant systemic circulation.