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Two days ago we compared Sculptra and Botox, two injectables that ask a patient to trust what goes under the skin. This issue is about what lands on top of it.

Contact dermatitis looks simple on paper: something touches skin, skin reacts, remove the something. In clinic it is one of the most frequently mismanaged rashes we see, because the something is rarely what the patient (or the chart) assumes.

About 20% of the general population is sensitized to at least one common contact allergen. In the most recent North American Contact Dermatitis Group (NACDG) data (2021 to 2022), 72% of patch-tested patients had at least one positive reaction, and nickel led the list at roughly 25%.

More than one in five of those patients reacted to something that was not on the standard 80-allergen screening series. The culprit is often hiding in plain sight: a wipe, a shampoo, a work glove, or the cream you prescribed.

Below are the five errors that generate most of the repeat visits:

The mechanism. Roughly 80% of contact dermatitis is irritant, not allergic. Irritant contact dermatitis is a direct chemical or physical injury to the barrier: soaps, solvents, frequent hand washing, friction. Anyone gets it with enough exposure, and it tends to burn or sting more than itch.

Allergic contact dermatitis is a delayed, T cell mediated response in a sensitized person. It typically appears 48 to 72 hours after exposure, itches intensely, often vesiculates, and can spread beyond the point of contact.

Why it matters. The two need different counseling. Irritant disease responds to barrier repair, exposure reduction, and gloves. Allergic disease does not improve until the specific allergen is identified and eliminated, and no amount of moisturizer fixes that.

Practical tip. Ask three questions: Does it burn or itch? Did it start within hours or a couple of days after exposure? Is it confined to the contact area or spreading? Burning, fast onset, and sharp margins point to irritant. Intense itch, delayed onset, and spread point to allergic.

The mechanism. Urushiol dermatitis is a classic delayed hypersensitivity reaction, and the immune response runs for two to three weeks regardless of how long you suppress it.

A short steroid burst (the 3, 5, or 6-day taper packs) stops before the reaction finishes, and the rash returns. The six-day methylprednisolone pack delivers about 84 mg total and is repeatedly described in the literature as insufficient for significant poison ivy.

The evidence. A claims-based analysis of poison ivy treatment found that shorter oral corticosteroid courses were associated with a 30% higher odds of a return visit. Despite recommendations for at least 14 days of treatment, most emergency clinicians prescribed shorter courses.

Practical tip. Mild, localized disease needs only a topical corticosteroid. For extensive disease, facial or genital involvement, or severe blistering, start prednisone at roughly 0.5 to 1 mg/kg (40 to 60 mg for most adults) and taper over 14 to 21 days. A complicated taper schedule is not required; the total duration is what matters.

Two myths to correct on the spot. Blister fluid does not contain urushiol and does not spread the rash. Unwashed clothing, tools, and pet fur do. And the rash that "keeps spreading" over several days is usually staggered exposure sites and different skin thicknesses reacting at different rates, not autoinoculation.

The mechanism. The products we reach for to treat a rash are themselves among the most common sensitizers. Topical antibiotics are the classic example: neomycin and bacitracin both sit in the NACDG top 20.

Topical corticosteroids sensitize too. NACDG data show about 4% of patch-tested patients react to at least one corticosteroid, with tixocortol pivalate (a marker for hydrocortisone and related class A steroids) and budesonide the most frequent.

Why it matters. A patient with a "steroid-resistant" or "antibiotic-resistant" eczema who keeps flaring on treatment may be allergic to the treatment. This is easy to miss because the steroid still partially suppresses the reaction, so the picture is a rash that improves slightly, never clears, and worsens the moment the tube runs out.

Practical tip. Stop reflexively adding bacitracin or triple antibiotic ointment to wounds and dermatitis; plain petrolatum performs as well for uncomplicated wound care and sensitizes no one.

If a dermatitis fails to clear on a topical steroid, switch to a different structural class (for example, from hydrocortisone to a class C steroid such as betamethasone valerate or desoximetasone) before escalating potency. Cross-reactivity within a class is common; across classes it is much less so.

The mechanism. Nickel is number one, and patients know it. So they blame the earrings, remove them, and keep flaring. Meanwhile the real allergen is in a product labeled "hypoallergenic," "natural," or "fragrance-free," none of which are regulated terms in the United States.

Where it hides. The 2021 to 2022 NACDG top allergens after nickel were methylisothiazolinone (a preservative in wipes, shampoos, dish soaps, and paints), hydroperoxides of linalool and limonene (oxidized fragrance components found in "natural" and essential oil products), cobalt, and methylchloroisothiazolinone/methylisothiazolinone. Propolis, a bee product marketed as natural skincare, has climbed steadily.

"Unscented" products can still contain masking fragrance. We covered the clean beauty version of this problem in a prior issue, and the pattern has not changed: natural is not the same as non-sensitizing.

Practical tip. Before referring, ask the patient to bring every product that touches the affected area, including hand soap at work, laundry detergent, hair products (scalp allergens often present on eyelids and neck, not scalp), and anything a partner uses.

Location is a clue: eyelids and neck suggest hair products or nail polish; hands suggest gloves, soaps, or occupational chemicals; a rash under a waistband or watch suggests metal or rubber accelerators.

The mechanism. Patch testing is the only way to confirm allergic contact dermatitis and identify the allergen. It is not a blood test and it is not a prick test; allergens are applied under occlusion and read at day 2 to 4 and again around day 7 to catch late reactions. Missing the second read misses a meaningful share of positives.

When to refer. The European Society of Contact Dermatitis guideline (updated 2026) lists two core indications: suspected contact allergy, acute or chronic, including occupational exposure, and any chronic dermatitis that is not improving with standard treatment. That second category is the one primary care most often delays on. If a "hand eczema" or "atopic flare" has had two adequate treatment trials and still is not controlled, the working diagnosis should be questioned.

Timing and preparation. Patch testing should be postponed during severe or generalized active dermatitis and while a patient is on high-dose systemic steroids. Get the flare under control first, then refer. Topical steroids and calcineurin inhibitors at the test site also blunt results, so ask the patient to keep the upper back clear of topicals for a week or two before the appointment.

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If this issue changes one thing in your practice, let it be the question you ask at the second visit: "what else touches this skin"

See you next week.

Until then, stay curious and keep translating science into realistic hope.

— The Derm for Primary Care Team