WALNUT CREEK, Calif. — Linda Piazza escaped adolescence with few pimples. As an adult, she wasn't so lucky.

It started when she was 30. Through with having her children and in a high-stress consulting job, Piazza began breaking out on her face. Over-the-counter products didn't help. A dermatologist prescribed an antibiotic that gave Piazza horrible panic attacks. Later, a day spa treated her acne with a chemical peel that made the raised, red bumps even worse.

"I was in tears," recalls Piazza, now 40, of Dublin. "At my age, I didn't understand why I had acne."

Piazza is not alone. According to Acne.org, 50 percent of adult women and 25 percent of adult men suffer from acne, the blockage of an oil gland that results in a blemish. They exhaust over-the-counter options and pop prescribed pills with unwanted side effects, to no avail.

While most acne is genetic, hormonal or related to stress, the oil chemistry of adult acne is more challenging than teen acne, explains Dennis Gross, M.D., a New York dermatologist and author of "Your Future Face."

"Adult oil is a greater source of nutrients to bacteria," Gross says. "That bacteria builds up and is then attacked by the immune system, causing the blemish to become more red and inflamed."

So it's even more crucial to find the proper treatments as an adult. It took Piazza a decade to find the right dermatologist and regimen — oral and topical — to clear up her skin while preventing future blemishes. That is the trick to treating acne, says her dermatologist, Dr. Jerome Potozkin of Walnut Creek, Calif.

The onset of adult acne is usually in the late 20s to early 30s. It is often more severe in women, who have a greater fluctuation of hormonal changes in adulthood, from weaning off birth control pills to pregnancy and menopause.

For men, the main cause is shaving, which can cause ingrown hairs and razor bumps that become infected.

Genetics and stress are factors for both genders. The relationship between acne and stress is a vicious cycle.

"Stress increases cortisol, which changes the chemistry of oil," Gross explains. "This change can then lead to acne, which definitely causes more stress — thus more cortisol."

Angry and unable to get clear, many adults experience depression and damaged self-esteem.

With so many products in a growing category of the $8.2 billion skin care industry, the choices can be daunting. The biggest problem: Many adults dry out their skin with use of oil-stripping products or excessive washing. This exacerbates the acne.

"The resulting dead skin adds more debris to an already clogged pore, congesting the complexion even more," Gross explains.

Dan Kern's acne started at age 11. In his case, it just never stopped.

By college, Kern had tried salicylic acid; Retin-A; clindamycin, a topical antibiotic; and sulfur to reduce hundreds of pimples on his face. All yielded minor results. The latter two actually made it worse.

"My skin plummeted to levels I had never seen," says Kern, now 34, of San Francisco.

By his early 20s, Kern had severe cystic lesions and pustules on his back.

"At my low point, I would not look at myself in the mirror," he said. "I just kept the light off in the bathroom and slept 14 hours a day."

He was eventually prescribed Accutane, which cleared him up completely. Five months later, however, the whiteheads resurfaced.

Frustrated, Kern recalled that benzoyl peroxide was the only treatment that consistently cleared his skin. Studies show that benzoyl peroxide kills 99.6 percent of bacteria on skin.

He added gentle washing and an oil-free moisturizer, and, sure enough, he zapped the zits forever.

Determined to help others, he studied the subject and 11 years ago founded Acne.org, where he talks to thousands of adult sufferers, some in their 60s. In 2004, backed by a St. Louis dermatologist, he wrote "Clear Skin: Heal Your Skin and End the Breakouts, Once and for All."

Kim Tyda had no luck with dermatologists, either.

Breakouts started last year, when she moved from Sacramento to Alameda, Calif. A dermatologist prescribed Differin. The ointment increased her acne.

Online, she read about similar reactions, and stumbled upon a thread on the Berkeley Parents Network about Face Reality Skin Care, an acne clinic in San Leandro, Calif. Laura Cooksey, the licensed aesthetician and acne expert there, got Tyda clear in eight weeks.

Cooksey treated Tyda's acne by building her up to a 10 percent benzoyl peroxide, staying one step ahead of the skin's adaptive process and adding exfoliants, antibacterials and peels along the way.

Many of Cooksey's clients come to her after popular products, such as Proactiv or Accutane, stop working, she says. Cooksey sees her patients every two weeks and clears 90 percent of them, usually within three months, she says.

While dermatologists disregard diet as a cause of acne, Cooksey is a believer. Iodides, or salt, and various hormones found in milk and peanuts contribute to acne, she says.

"It's not the oil in French fries, it's the salt," she explains.

Cooksey also sees cases of acne cosmetica, which is caused by pore-clogging oils, such as lanolin and cocoa butter, in makeup. She custom blends a mineral makeup for her acne-prone clients. Isopropyl myristate, a common ingredient in makeup, and, ironically, the cream form of Retin-A, also clogs pores.

"You'd want to give the cream form to the anti-aging client because it penetrates the skin," Cooksey says. "But you'd never want to give it to an acne-sufferer."

Therein, perhaps, is the benefit of having acne as an adult.

"Most exfoliants — glycolic, salicylic and lactic acids, and vitamin A — are great for both acne and anti-aging," Cooksey says.

Dolores Settera is 73. She's had acne since she was 14 and recalls the days of tar soaps and X-ray treatments.

"It's kind of been a blessing because I haven't had to deal with wrinkles and stuff," says Settera, of Oakland, Calif.

Acne keeps her young in spirit, too.

"My husband and I had reservations for our 30th anniversary last year and I looked like a unicorn with a pimple right on my nose," she recalls, laughing. "I guess there are some things you just don't outgrow."

<b>CAUSES OF ACNE</b><b>Women</b>• Hormonal fluctuations: Oil chemistry changes post-pregnancy, pre-menses and post-menopause. Stopping use of birth control pills can trigger acne.• Acne cosmetica: Pore-clogging powders and other cosmetics lead to blackheads and other flat acne.<b>Men</b>• Improper shaving: Causes ingrown hairs that block oil glands and become infected.• Poor product selection: Harsh soaps dry out skin; resulting dead skin adds more debris to already clogged pores.<b>Both</b>• Stress: Increases cortisol, which changes oil chemistry. This change can lead to acne.• Genetics: Acne is a hereditary disease of the pores.• Mechanics: "Cell phone" acne on the chin; acne from repetitive touching of the skin or wearing hats or helmets.• Diet/environment: Some experts link acne to diets high in salt, changes in humidity and high levels of calcium or magnesium in a city's tap water.Sources include Dennis Gross, M.D.; and Julie Anusevicius, Face Reality Skin Care<b>TREAMENTS</b>Solutions vary depending on the cause, severity and type of acne. Remember to wash the face with bare hands and a gentle cleanser and follow with an oil-free moisturizer containing sunscreen.<B>Over-the-counter</B>• Always start here. The two antibacterial products are salicylic acid and benzoyl peroxide. Studies prove the 2.5 percent version of the latter is as effective as 5 percent, without the over-drying.<B>Topical Prescription</b>• If over-the-counter products don't help, it's time to see a dermatologist. He or she will most likely prescribe a topical gel that has two antibacterials in it, usually benzoyl peroxide and clindamycin. If that doesn't work, he or she may try a retinol-based ointment such as Differin or Retin-A, plus an oral antiobiotic. Accutane is prescribed for cystic acne. Most of these treatments cause acne to worsen before it gets better.<b>Exfoliants</b>• Serums and peels containing exfoliating ingredients such as vitamin C or glycolic, salicylic or lactic acids can be administered by a dermatologist or licensed aesthetician on a regular basis. Some experience irritation with this treatment.<b>Other Option</b>• Laser- and light-based therapies are used on those who see little or no results with traditional therapies. Injectable steroids can also be used to spot reduce a pronounced blemish in a matter of days.Sources include Jerome Potozkin, M.D.<b>ACNE MYTHS</b>• Acne is not only a teenager's skin disease.• Over-washing your face does not prevent acne; it can exacerbate it.• The jury's still out on diet and acne; to be safe, lower your iodide intake.• Drink lots of water and get plenty of vitamin B.• Wash your face after exercise.• Spot reduction is not as effective as treating the whole face.• Flat acne (blackheads) and inflamed acne (whiteheads, cysts, pustules) should be treated differently• If left untreated, noninflammatory acne can become inflamed.Sources include Dennis Gross, M.D.; and Laura Cooksey, Face Reality Skin Care