One reasonable and safe strategy would be to use a cosmetic acid nightly for six to 12 months (depending on concentration), and thereafter only once per week. Another very effective strategy recommended by some doctors is alternate use of a cosmetic acid and tretinoin.

See for example "Retinoid Therapy: Compatible Skin Care," Skin Pharmacology and Applied Skin Physiology, 12(3), 111-119, 1999 May-Jun (abstract available on Medline.) While this study used tretinoin at night and acid during the day, it is best to only apply the products on alternate nights to reduce sun-sensitivity. This combination strategy is believed to be more effective than using either product alone. Application frequency or concentration of the products may have to be reduced if you get irritation, peeling or redness.

In any case, after about 12 months, application frequency should be decreased to once per week for the acid and twice per week for tretinoin. Whatever schedule you use with cosmetic acids, if your skin looks red and irritated during the day, simply decrease the application frequency, use a lower-concentration product, or dilute it with distilled water (if it is water-based).

When using Alpha-Hydroxy Acids (AHAs) daily or alternating with tretinoin, most people will need to limit concentration to 12% or less. If you use this strategy with salicylic acid, select a product with a concentration under 5% to avoid potential toxicity. Higher-concentration salicylic acid products usually aren't practical for long-term use anyway, because they often cause a lot of crusting and peeling of the skin.

AHAs and salicylic acid produce different effects on pore appearance. Contrary to some advertising, AHAs are not a treatment for large pores and do little to dissolve sebum-keratin plugs in the pores. In fact, peeling off the upper epidermal layers with AHAs may temporarily make your pores look slightly larger.

Salicylic acid, on the other hand, can help dissolve sebum-keratin plugs that clog pores. That is why it is used in some acne medications like Neutrogena's Clear Pore Treatment (with 2% salicylic acid). There is some research suggesting that the ability of salicylic acid to penetrate into pores may make it more effective than AHAs of comparable acidity for treating photodamage.

If you have a problem with clogged pores, you can use the combination strategy of alternating nightly use of 2%-4% salicylic acid and tretinoin. The 56 gram, 2% Neutrogena Clear Pore Treatment Gel is the most widely available salicylic acid product; it costs about $6. You can find other products with 3% or 4% concentration at specialty cosmetics stores or on the Net. Avoid salicylic acid products that contain a high percentage of alcohol, which is not good for your skin.

Use Cosmetic Acids at Night

You should only use cosmetic acids at night, since they cause sun sensitivity. It is very important to always use sunblock when outside while using cosmetic acids, or you will defeat any benefits you gain by allowing more sun damage.

Don't apply cosmetic acids with any other products since the acids can react with other chemicals. Don't apply them right next to your eyes. The first time you try a cosmetic acid, do it on a night when you won't have to work the next day; otherwise, you could wake up with a very red face. Start out with a low concentration, or dilute the acid and work your way up to full concentration gradually.

Also, stand next to a sink when you apply the acid so you can flush your eyes and face with water if you get any in your eyes or if your face starts to sting badly. (There will be an initial mild stinging sensation that subsides quickly.) Also be aware that cosmetic acids may lighten your skin slightly with long-term use.

There are some sources for high-concentration AHA and salicylic acid products, but they may not be safe for general home use. These are similar to the solutions used by doctors for light and medium chemical peels. However, using high-concentration acid solutions without specialized knowledge is very dangerous.

Some of these products may not be medical-grade, and might contain impurities or toxins that could damage your skin. Some high-concentration solutions can only be left on the skin for a short time and then must be quickly washed off or neutralized with a base solution to prevent burns, scarring, and possible infection.

It only takes minutes for some solutions to penetrate into the dermis and cause permanent scars. They can also cause damage if they get into your eyes. So unless you have specialized medical knowledge on how to use high-concentration acids, stick with AHA products under 20% concentration or salicylic acid under 7% available from reputable stores or from cosmetologists and doctors. If you want to get light or medium resurfacing, have it done by someone with experience. Don't risk scarring your face.

Most cosmetic acids are not stamped with expiration dates, but manufacturers say they start to degrade after about three years. It is best to only buy one tube at a time to assure potency.