Coal tar

Coal tar has been used on skin since the nineteenth century, which makes it older than most of the regulatory apparatus around it. It survives in the drug regulations because it works, and its entry there does something no other ingredient on this site does.

The regulation that demands a number

Most of this site is about labels that will not tell you how much of something is present. Coal tar is the exception, and the wording is worth reading.

The regulation permits “coal tar, 0.5 to 5 percent”. Then: “When a coal tar solution, derivative, or fraction is used as the source of the coal tar, the labeling shall specify the identity and concentration of the coal tar source used and the concentration of the coal tar present in the final product.”

Two concentrations, both compulsory. If a product uses a coal tar solution rather than crude tar, the label must name that solution, say how concentrated it is, and separately say how much actual coal tar ends up in the product.

The reason is that coal tar is not one substance. It is a complex mixture whose composition varies with the source, and a 5 percent coal tar solution and 5 percent coal tar are not remotely the same thing. Without that rule the number on a pack would be meaningless. With it, this is one of the few products where the strength is genuinely comparable between brands.

Where it sits against the alternatives

Pyrithione zinc is the modern default and it is easier to live with. Coal tar smells, it stains fabric and light hair, and it is photosensitising, so sun exposure after use is worth avoiding.

Coal tar is generally kept for cases where the simpler actives have not worked, and it retains a real place in psoriasis in particular. That is a clinical decision rather than a shelf decision, which is worth saying because the products are sold next to each other without any indication that one is a step up.

Using it without wrecking the hair

The dermatology guidance for textured hair applies to tar with more force than to anything else in the category, because it is the most drying of the group.

Apply it to the scalp, not the hair shafts. Do not leave it on longer than directed, because “prolonged contact time is not needed for medicated shampoos to be effective”. Use it carefully on fragile hair, which is what the guidance advises for tar, selenium sulfide and zinc pyrithione alike.

For hair that already breaks easily, running a tar shampoo down the length is a reliable way to trade a scalp problem for a hair problem. The wider version of that trade is on the curly and coily hair page.

The carcinogenicity question, in proportion

Coal tar is a complex mixture containing polycyclic aromatic hydrocarbons, and occupational exposure to coal tar in industrial settings is associated with cancer. That is well established and it is not in dispute.

The therapeutic use is a different exposure: a defined concentration, on skin, washed off, intermittently. Decades of dermatological use have not produced a clear signal at those exposures, and the ingredient remains in the monograph.

This is the hazard and exposure distinction in its textbook form, and it is set out at length in the guide on hazard and exposure. The honest summary is that the concern is real in one setting, has not been demonstrated in the other, and that the regulation reflects that rather than ignoring it.

Reading the pack

Look for the drug facts panel. An anti-dandruff product is a drug, so it has one, and the active and its strength are stated there rather than in the ingredient list.

On products using a solution rather than crude tar, expect to see both numbers. Names to recognise include coal tar solution, liquor carbonis detergens and LCD, all of which are the processed form rather than the tar itself.