What is the role of coal tar shampoo in treating scalp psoriasis?

Coal Tar Shampoo for Scalp Psoriasis

Coal tar shampoo is an effective, time-tested treatment option for scalp psoriasis that can be used as monotherapy or combined with other topical therapies, though it is often limited by cosmetic concerns including staining, odor, and messiness. 1

Efficacy and Clinical Evidence

Coal tar has been used for over 100 years in psoriasis treatment and demonstrates meaningful clinical benefit 1:

• In a randomized controlled trial of 324 patients with mild to moderate psoriasis, 1% coal tar lotion showed better improvement in both PASI score and Total Severity Score compared to 5% coal tar extract 1
• A newer cosmetically acceptable LCD (liquor carbonis distillate) 15% solution demonstrated 58% mean reduction in PASI scores versus 37% with calcipotriene cream at 12 weeks, with better maintenance of improvement after treatment withdrawal 2
• Coal tar shampoos containing 2-10% coal tar solution are considered effective for scalp psoriasis, though double-blind studies specifically for shampoo formulations are lacking 3

Formulations and Dosing

Multiple formulations exist, with shampoo being particularly suited for scalp involvement 1:

• Coal tar shampoos typically contain 2-10% coal tar solution 3
• Available in both over-the-counter and prescription-strength formulations 1
• Can be used as monotherapy or combined with topical corticosteroids 1
• Newer foam formulations have been developed to improve cosmetic acceptability and ease of application 4

Mechanism and Additional Benefits

Coal tar functions as an antiproliferative and anti-inflammatory agent, with particularly potent efficacy against pruritus 1:

• Acts as an aryl hydrocarbon receptor agonist 1
• The exact mechanism of action remains unclear 1
• The anti-pruritic effect is an important clinical feature that distinguishes coal tar from other topical treatments 1

Adverse Effects and Tolerability Issues

The primary limitation of coal tar is poor patient tolerability due to cosmetic concerns rather than safety issues 1:

Common adverse effects:

• Staining of skin, clothing, and bathing surfaces 1
• Tar odor present in almost all products 1
• Irritant contact dermatitis 1
• Folliculitis 1, 5
• Photosensitivity to UVA 1
• Contact sensitivity (reported in several cases) 5

Important safety consideration:

While coal tar is carcinogenic in animals, there are no convincing data proving carcinogenicity in humans, and epidemiologic studies fail to show increased risk of skin cancer in patients who use coal tar 1, 5

Special Populations

Pregnancy:

The risk of topical coal tar used for brief periods during pregnancy is likely to be small 1

Pediatric use:

• Use with caution in children 1
• Coal tar preparations can be used as monotherapy or combined with other topical therapies for pediatric psoriasis (Strength of recommendation: C) 1
• When combined with phototherapy in children, there is a theoretical increased risk of carcinogenicity with prolonged use; this risk may be decreased by alternating with other treatment modalities 1

Practical Application Strategy

For scalp psoriasis, coal tar shampoo should be considered as a first-line or second-line option, particularly when:

• Corticosteroid-sparing therapy is desired 2
• Pruritus is a prominent symptom 1
• Patient can tolerate cosmetic inconveniences 1
• Long-term maintenance therapy is needed 3

Common pitfall to avoid: Starting with concentrations higher than necessary—evidence suggests no benefit from exceeding 5% crude coal tar concentration, as the dose-response curve plateaus between 1-5% 6

Combination Therapy

Coal tar can be effectively combined with topical corticosteroids to enhance efficacy 1, 3:

• This combination approach may improve patient outcomes while potentially reducing corticosteroid exposure 2
• Coal tar is commonly used with phototherapy (Goeckerman treatment), though this increases theoretical carcinogenicity risk with prolonged use 1