If your “dandruff shampoo” isn’t touching the flakes, or if the scale creeps beyond your hairline, you may be dealing with scalp psoriasis, not simple dandruff (which is the lay term most people use for seborrheic dermatitis). The two are common, often itchy, and can overlap—but they aren’t the same disease and they’re treated differently. Here’s how to tell them apart, what’s happening under the skin, and what actually works.

Quick definitions

• Dandruff / Seborrheic dermatitis (SD): An inflammatory rash driven in part by skin oils and the Malassezia yeast that thrives in oily areas (scalp, eyebrows, sides of the nose, ears, chest). Flakes are usually fine, white-to-yellow, and greasy, with underlying redness and itch.

• Scalp psoriasis: An autoimmune, inflammatory condition driven by the IL-23/IL-17 immune pathway, causing well-demarcated, thick plaques with silvery-white scale that often extend beyond the hairline and may show up elsewhere on the body (elbows, knees, low back). Nails may pit or lift.

Why they happen (pathophysiology in plain English)

• Seborrheic dermatitis: Your skin’s oil + Malassezia yeast + an overzealous local immune response = scaling and redness. Think “oily + yeast-reactive skin.” This is why antifungal shampoos help.

• Psoriasis: A systemic immune disorder—T cells and cytokines (notably the IL-23/IL-17 axis) push skin cells to turn over too fast, piling up into thick plaques. These same inflammatory signals also link psoriasis to other health risks.

How common are they?

• Psoriasis affects roughly 3% of U.S. adults (~7.5–8 million people). Scalp involvement occurs in up to half of those with psoriasis.

• Seborrheic dermatitis affects about 1–5% of adults worldwide; milder “dandruff” (flaking without visible inflammation) is far more common.

Why the distinction matters

1. Different treatments: Antifungals and keratolytics are first-line for dandruff; psoriasis may need topical steroids, vitamin-D analogs, phototherapy, or even biologics if moderate–severe.

2. Systemic risks: Psoriasis isn’t just skin-deep. It’s associated with psoriatic arthritis (~30% lifetime risk) and elevated cardiometabolic risk (MI, stroke), especially with more severe disease—screening matters.

What actually works (evidence-based care)

If it’s mostly dandruff / seborrheic dermatitis

• OTC shampoos, used correctly (let sit 3–5 minutes before rinsing), 3–4x/week at first:

  • Pyrithione zinc 1%

  • Selenium sulfide 1%

  • Ketoconazole 1% (2% by Rx)

  • Coal tar or salicylic acid for scale lift

• Short courses of topical corticosteroids (lotion/foam/solution) for flares, especially on inflamed facial or ear areas—avoid long-term daily use to minimize skin thinning.

If it’s scalp psoriasis

• Topical corticosteroids (medium–high potency solutions/foams/sprays) are first-line for plaques; combine or alternate with vitamin-D analogs (calcipotriene) to maintain control.

• Keratolytics (salicylic acid) can help lift heavy scale so medications penetrate.

• Phototherapy (targeted UVB) and systemic options for refractory disease: methotrexate, cyclosporine, acitretin, or biologics targeting TNF-α, IL-17, IL-23.

• Expect rapid improvement with IL-17 inhibitors and robust durability with IL-23 inhibitors (class effects seen in trials).

When it’s both (sebopsoriasis)

• Practical sequence that often works:

  1. Ketoconazole or pyrithione-zinc shampoo,

  2. Topical steroid foam/solution,

  3. Add vitamin-D analog for maintenance.

Red flags & when to see a professional

• Scale extends past the hairline, or plaques appear on elbows, knees, lower back, genitals, or inside ears.

• Nail changes: pitting, lifting, crumbling, or a yellow-brown “oil drop” discoloration.

• Joint symptoms: morning stiffness, swollen fingers/toes, heel pain—possible psoriatic arthritis.

• Significant redness, pain, or swelling of the face or eyelids, or failure of OTC care after 4–6 weeks.

Step-by-step home approach (what I tell my patients)

1. Look closely at the pattern. Fine greasy flakes = seborrheic dermatitis; thick silvery plaques beyond the hairline = psoriasis.

2. Try the right shampoo—consistently. Rotate ketoconazole, selenium sulfide, and pyrithione zinc for 2–4 weeks if SD is likely.

3. If plaques are thick or extend beyond the hairline, ask your clinician about topical steroid solution/foam ± calcipotriene.

4. Reassess at 4–6 weeks. If persistent plaques, nail changes, or joint symptoms → see dermatology.

Bottom line

• Dandruff/seborrheic dermatitis = oily/yeast-associated inflammation, fine greasy flakes, responds to antifungal shampoos.

• Scalp psoriasis = immune-driven plaques with thick silvery scale, often beyond the hairline and with body or nail findings; may require prescription or systemic therapy and carries systemic health implications.

References

1. American Academy of Dermatology. Psoriasis: Overview and Treatment.

2. American Academy of Dermatology. Seborrheic Dermatitis: Diagnosis and Treatment.

3. Mayo Clinic. Scalp Psoriasis vs. Dandruff.

4. DermNet NZ. Psoriasis – Scalp, Nails, and Sebopsoriasis.

5. Armstrong AW, Read C. Pathophysiology, Clinical Presentation, and Treatment of Psoriasis: A Review. JAMA. 2020;323(19):1945-1960.

6. Parisi R, et al. Global Epidemiology of Psoriasis: A Systematic Review. J Invest Dermatol. 2013;133(2):377–385.

7. National Psoriasis Foundation. Psoriatic Arthritis Overview.

8. Gupta AK, et al. Seborrheic Dermatitis. Lancet. 2004;364(9440):326–333.

9. Nestle FO, Kaplan DH, Barker J. Psoriasis. N Engl J Med. 2009;361:496–509.

10. Menter A, et al. Joint AAD-NPF Guidelines of Care for the Management of Psoriasis with Biologic Therapies. J Am Acad Dermatol. 2019;80(4):1029–1072.