how come testosterone gels applied to the skin absorb into the bloodstream but applying a product like selsum blue to your skin doesn’t? Or does it? ELI 5

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how come testosterone gels applied to the skin absorb into the bloodstream but applying a product like selsum blue to your skin doesn’t? Or does it? ELI 5

In: Biology

Molecules that are hydrophobic (fat-soluble) can pass through biological membranes more easily, including the skin. Steroid hormones (testosterone and cortisone) and certain vitamins (A and D) are fat-soluble. So they can pass through the skin.

Molecules that are hydrophilic (water-soluble) cannot pass through the skin very easily. I think sunscreen is mostly water and alcohol, plus a few ions (zinc or titanium) to reflect UV-rays. So it doesn’t pass through the skin.