What is the recommended regimen, safety precautions, and alternative therapies for using coal‑tar (2–5% shampoo or ointment) in a patient with mild‑to‑moderate scalp psoriasis?

Coal Tar for Scalp Psoriasis

Coal tar preparations (2-5% shampoo or ointment) are recommended as an effective treatment option for mild-to-moderate scalp psoriasis, with a strong evidence base (Grade A recommendation), though they are typically considered after or alongside more cosmetically elegant first-line options like calcipotriene/betamethasone combination therapy. 1, 2

Recommended Treatment Regimen

Standard Application Protocol

• Apply coal tar shampoo (2-10% coal tar solution) or ointment to affected scalp areas for 8-12 weeks as the recommended treatment duration for mild-to-moderate psoriasis 1, 3
• Coal tar shampoo formulations are most practical for scalp application, as crude coal tar application directly to the scalp is difficult 4
• A newer 1% coal tar lotion formulation has demonstrated superior efficacy compared to conventional 5% coal tar preparations, with statistically significant improvement in Total Sign Score (-10.6% difference, p=0.04) 5

Combination Therapy Options

• Coal tar can be combined with phototherapy (Goeckerman therapy) consisting of coal tar application followed by narrowband UVB exposure, which has sufficient evidence for efficacy (Grade B recommendation) 1, 3
• Apply coal tar 1 day before phototherapy for optimal benefit; avoid application immediately before phototherapy as this causes tar pigmentation 1
• Coal tar can be used alongside topical corticosteroids for enhanced efficacy 1

Mechanism and Efficacy

• Coal tar decreases keratinocyte proliferation by suppressing DNA synthesis, suppresses inflammation, and has potent anti-pruritic effects through aryl hydrocarbon receptor binding 1, 4, 6
• Multiple clinical trials and systematic reviews have demonstrated efficacy in psoriasis treatment 1

Critical Safety Precautions

Common Adverse Effects

• Local irritation, folliculitis, contact dermatitis, and phototoxicity are the most frequently reported adverse effects 1, 3
• Cosmetic issues including clothing staining and characteristic tar odor significantly reduce patient adherence 1, 3
• Phototoxicity occurs due to polycyclic aromatic hydrocarbons acting as photosensitizers 3

Pregnancy and Lactation

• Avoid coal tar preparations during pregnancy and lactation as recommended by the American Academy of Dermatology 1, 3
• While retrospective human studies show no adverse fetal effects, animal studies with large doses demonstrated increased risk of cleft palates, small lungs, and perinatal mortality 1, 3

Carcinogenicity Concerns

• Possible carcinogenicity remains controversial but not proven in dermatologic use 1
• Dermatologic studies on topical preparations have not revealed increased cancer risk, though occupational exposure studies (coal tar industry, road construction) document carcinogenicity with prolonged exposures over many years 1, 3
• When using coal tar with phototherapy in pediatric patients, theoretical long-term carcinogenesis risk exists; consider alternating with other treatment modalities 1

Application Site Restrictions

• Avoid application to the face or highly visible areas due to staining and irritation concerns 7
• Coal tar is rarely used on intertriginous areas due to poor tolerability 1

Alternative and First-Line Therapies for Scalp Psoriasis

Preferred First-Line Option

• Calcipotriene 0.005%/betamethasone dipropionate 0.064% combination scalp solution applied once daily represents the most effective first-line approach with rapid onset of action and improved compliance 2, 8
• This combination achieves 48-74% of patients reaching clear or almost clear status 2

Other Topical Alternatives

• Topical corticosteroids are the most frequently used treatments for scalp psoriasis, with maximal efficacy reached within 3-4 weeks; clobetasol propionate 0.05% shampoo is safe and efficacious for long-term management 4, 8
• Salicylic acid 5-10% has pronounced keratolytic effect and should be formulated in an easily washable ointment 4
• Never combine salicylic acid with calcipotriene simultaneously as the acidic pH completely inactivates calcipotriene 2, 7
• Vitamin D3 analogues (calcipotriol, tacalcitol) inhibit epidermal proliferation and inflammation with substantial antipsoriatic effect; maximum 100g per week to avoid hypercalcemia 2, 4

When to Escalate Treatment

• Consider systemic therapy (methotrexate, cyclosporine, acitretin) or phototherapy if: symptomatic disease persists, more than minimal impact on quality of life, inadequate response to topical therapy after 8-12 weeks, or recalcitrant disease 2, 4

Common Pitfalls to Avoid

• Do not apply coal tar immediately before phototherapy as this causes problematic tar pigmentation; apply 1 day prior instead 1
• Warn patients about staining of skin, clothing, and bathtub/shower to improve adherence 1, 3
• Avoid high-potency corticosteroids on the scalp beyond 4 weeks of continuous use without transitioning to intermittent dosing 2
• Do not use coal tar as monotherapy when more cosmetically acceptable options (calcipotriene/betamethasone combination) are available and appropriate, as coal tar's cosmetic drawbacks significantly impact adherence 1, 8