Coal tar compared against its own cream base

Eighteen people with psoriasis applied 5% coal tar solution to plaques on one side of the body and the identical cream base without the tar to matching plaques on the other, for four weeks, with neither they nor the assessors knowing which was which. The plain base produced a 35% mean improvement. The base with coal tar produced 48.7%. The difference was statistically significant. This is the single most informative number available on coal tar, and it does two things at once. It confirms the tar itself contributes a real effect beyond simply moisturising the skin. And it shows how much of the apparent benefit of any tar product comes from the emollient it is carried in — most of it. Nearly every other coal tar study compares it against a different treatment rather than against its own base, which is why this small trial carries so much weight.