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According to the appointment book of Ava Shamban, M.D., the dermatologist founder of AVA M.D. MedSpa in Los Angeles, "some facial irritation" was the reason a 40-something woman I'll call Joyce came to see her. "It turned out the patient had a mild chemical burn," recalls Shamban. The surprise was that it was neither an allergic reaction nor the by-product of a chemical peel or laser procedure. The burn was self-inflicted: To ramp up her skin-care regimen, Joyce had combined products, not realizing they could react adversely.

Joyce divulged that she had been seeing two dermatologists. One treated her with retinol, a vitamin-A derivative that improves skin structure but can cause dryness, and the other recommended alpha hydroxy acids, which also exfoliate. Neither doctor knew about the other. "The patient used the high-concentration products together and then came to me to treat the burn that resulted!" says Shamban, who has seen this kind of medicine-cabinet mixology before.

The thinking is, if a little product is good, then a lot is better, says Kim Logan, the business vice president for Dermstore.com, which sells skin-care and cosmeceutical products. That's why purchasing some Dermstore products, such as Green Cream 9, which comes in two lower concentrations, requires a phone consultation with a company skin-care consultant before the customer can check out. "We ask about their familiarity with retinol, what else they're using, and so on before we give callers a password to purchase it," explains Logan.

Boston dermatologist Ranella Hirsch, M.D., the president of the American Society of Cosmetic Dermatology and Aesthetic Surgery, concurs with Logan's analysis. "Most commonly, you see people overdoing scrubs or alpha hydroxy acids in conjunction with a retinoid [the prescription-strength version of retinol]. Or just using too much retinoid—this is by far the most prevalent."

So besides retinoids, which ingredients or combinations should be avoided?

• Alpha hydroxy acids with anything, according to Hirsch, who extends the ban to any product with "an active ingredient that needs to be in a certain acid-base balance"
• Salicylic-acid treatments and scrubs within a 24-hour period
• Vitamin-C serums and benzoyl peroxide spot treatments for pimples
• At-home peels or pore-cleansing masks with products that contain a high concentration of strong essential oils, particularly eucalyptus and lemon
• Waxing with vitamin C, retinoids, or Accutane

But you don't even have to keep this list handy if you take a piece of advice from Hirsch and Shamban: Stop being a skin-care chemist and go back to the basics.

Shamban recommends this daytime regimen: nonsoap cleanser, low-strength AHA moisturizer with antioxidants, and sunscreen. In the evening, use the same cleanser (or for oily skin, one with salicylic acid) and a retinol-and-peptide cream. Hirsch boils skin care down to three essentials: a moisturizing cleanser, a moisturizer with good broad-spectrum sun protection, and a prescription retinoid at night. Both dermatologists point out that retinol and retinoids should be used only at night because they break down in sunlight and night is when your skin recovers.

The goal is to calm and protect, not irritate or inflame. The lesson here: Slow, steady—and simple—wins the race.