Dermatologist's Take
Coal tar is a by-product of heating coal, and it has been used on skin for well over a century. Its best-evidenced use is plaque psoriasis, where the US dermatology guidelines still give it a top-grade recommendation. The honest picture underneath that grade is more mixed: there is only one properly controlled trial comparing tar with its own cream base, and it found a real but modest advantage — 49% improvement with tar against 35% for the base alone. Against modern vitamin D creams, tar works more slowly, but in three of five head-to-head trials the difference had closed by 8 to 12 weeks, and tar irritated less. The cancer question deserves a straight answer: coal tar contains carcinogenic compounds, and industrial coal tar exposures are classified as causing cancer in humans. But studies following dermatology patients — including 13,200 people over a median of 21 years — have not found an increased cancer risk from medical use. What actually limits coal tar is none of this. It is that it smells strong, stains clothes, hair and bathtubs, and can cause a stinging reaction in sunlight.
At a glance
- AKA pix lithantracis, LCD, liquor carbonis detergens, coal tar solution
- 0.5–5% actual coal tar in US over-the-counter products; 2–10% crude tar in hospital regimens
- Works with salicylic acid, moisturizers, UVB phototherapy. Do not combine with UV unsupervised
- Evidence: Moderate for plaque psoriasis · Traditional for dandruff and seborrheic dermatitis
What is it?
Coal tar is what is left when coal is heated without air, a process originally run to make coke and coal gas. It was industrial waste before it was a medicine.
It is not a single substance. It is a mixture of thousands of compounds, only a few hundred of which have ever been identified, and its exact make-up varies with the coal it came from and how hot the process ran. That is why standardising it is genuinely difficult, and why comparing one product with another is harder than it looks.
You will meet it in several forms:
- Crude coal tar — unrefined, mixed into an ointment base. This is the material used in hospital psoriasis programmes.
- Coal tar solution, also called LCD — an alcohol extract of tar. Nominally 20% tar, though the real tar content is lower because insoluble parts are filtered out.
- Refined or "solubilised" extracts — fractionated to remove the heaviest compounds, which cuts the smell and staining. This is what most shampoos contain.
A practical warning about labels. A shampoo advertising "2% tar" might contain 2% actual coal tar — or it might contain 0.4%, because the 2% refers to an extract that is itself only a fifth tar. In the US the label has to state both figures, so look for the smaller number in the active ingredients box, not the marketing on the front.
How it works
Coal tar works in at least two ways, one long known and one only recently explained.
It slows down overactive skin cell production. In psoriasis, skin cells multiply far too fast, which is what builds the thick scaly plaques. Tar suppresses that, and it does so more strongly when combined with ultraviolet light — which is the basis of the century-old Goeckerman regimen, tar plus UVB.
It switches on a receptor that rebuilds the barrier. This is the interesting modern finding. In 2013, researchers showed coal tar activates something called the aryl hydrocarbon receptor in skin cells, which in turn restores production of filaggrin — the protein that is deficient in many people with eczema. When they silenced that receptor, the effect vanished entirely. A later study from the same group showed the same pathway also raises the skin's own antimicrobial peptides and shifts the balance of bacteria living on the skin.
That work is genuinely elegant and it explains a two-thousand-year-old remedy in modern molecular terms. It is worth being clear what it is not, though: it was done in laboratory-grown skin models and small groups of volunteers, measuring proteins and bacteria rather than how much better anyone's eczema got. It has never been turned into a modern treatment trial.
Subtypes
Where coal tar is genuinely supported, and where it is mostly tradition.
- Plaque psoriasis — the best-evidenced use. US dermatology guidelines recommend it for mild to moderate psoriasis. One properly controlled trial and five comparisons against vitamin D creams back it up.
- Scalp psoriasis — widely used, but the evidence is weaker than the practice. A large Cochrane review could not generate any pooled estimate for tar. UK guidelines place it second-line and specifically advise against using tar shampoo alone for severe scalp psoriasis.
- Dandruff and seborrheic dermatitis — permitted by US regulators, but this reflects long-standing use rather than a body of trial evidence. In the one randomised comparison available, a 0.5% tar shampoo was clinically about equal to a non-tar shampoo, but the non-tar shampoo was significantly better at reducing the yeast involved.
- Eczema — traditional. The strongest support is the laboratory mechanism work. One 30-person, four-week trial found a purified tar cream about equal to 1% hydrocortisone, the mildest steroid available. US eczema guidelines declined to make a recommendation on tar in 2014 and dropped it entirely from the 2023 update.
- Acne and fungal infections — US regulators have specifically ruled coal tar not effective for these. Do not use it for them.
Concentrations
United States, over the counter: 0.5% to 5% actual coal tar. That single range covers dandruff, seborrheic dermatitis and psoriasis — there is no higher allowance for psoriasis.
Hospital regimens: crude coal tar at 2%, 5% or 10%, applied under occlusion for four to five hours a day alongside UVB light. This is not something to reproduce at home.
Prescription: there is no FDA-approved prescription coal tar product currently on the US market. Higher strengths exist only through compounding pharmacies.
Reading the label. Because coal tar extracts are diluted, the same-looking number can mean very different things:
- A product listing "Coal Tar 0.5% (2% solubilised coal tar extract)" contains half a percent of actual tar.
- A product listing "Coal Tar 2% (from Coal Tar Topical Solution USP, 10%)" contains two percent.
Both could be advertised as "2%." That is a four- to fivefold difference in what actually reaches your skin.
Conditions it treats
What to expect
Improvement is slow. In the trials, tar took four weeks to show a significant effect where a vitamin D cream showed one at two weeks. Give it eight to twelve weeks before judging.
The practical burden is the real story, and it is why most people stop.
- The smell. Distinctive, strong, and the most common complaint by a wide margin.
- Staining. Skin, clothing, bedding, towels and bath surfaces. It can also discolour hair, which matters most if your hair is blonde, grey or chemically treated.
- Folliculitis — small acne-like bumps in the treated area, sometimes called "tar acne." In a 21-year hospital series of children on tar plus light therapy, this affected 42%, and it was the only side effect recorded.
- Stinging in sunlight. Coal tar causes a specific, immediate burning sensation on UVA exposure, quite different from a delayed sunburn. US labels carry a warning that tar may increase your tendency to burn for up to 24 hours after applying it. Sunscreen blocks it.
Standard label advice: do not use it for prolonged periods without medical advice, do not combine it with UV light or prescription psoriasis treatments unless a clinician has directed it, and keep leave-on forms away from the genital and anal area unless a doctor has advised otherwise. Avoid broken, blistered, raw or infected skin.
Irritation and contact allergy occur but are not common.
The evidence3 studies
Studies behind the treatments above. Tap any tag to see what it means.
5% coal tar solution improved psoriasis by 49% over four weeks, against 35% for the identical cream base without tar.
Moderate evidence
Randomised, double-blind, and — unusually for coal tar — compared against its own emollient base rather than against nothing. That design is what makes it valuable. Graded moderate rather than strong only because 18 people completed it.
18 people
How many people took part. Bigger studies are less likely to show a result by chance alone.
Coal tar used as a skin treatment was not associated with increased cancer risk in 13,200 patients followed for a median of 21 years.
Moderate evidence
A large historical cohort with long follow-up and a sensible comparison group — patients treated with steroid creams only. It is observational rather than randomised, and the authors are explicit about several gaps, which is why it is graded moderate rather than strong.
13,200 people
How many people took part. Bigger studies are less likely to show a result by chance alone.
One skin cancer type missing
Basal cell carcinoma, the most common skin cancer, is not recorded in the national registry used and is therefore absent from the analysis. Median tar exposure was also only six months.
Coal tar restores filaggrin — the barrier protein missing in many people with eczema — by switching on a receptor called AHR.
Moderate evidence
A mechanistic study in laboratory-grown skin models built from patients' own cells, with a knockout control that removed the effect entirely. That control is what makes the finding causal rather than merely associated. It is not evidence that coal tar improves eczema in practice.
Laboratory study
Three-dimensional skin equivalents grown from the skin cells of eczema patients and healthy controls, plus tissue samples from patients. The endpoints are proteins and gene expression, not symptom scores.