Retin-A comes in a cream, liquid or gel. In liquid form, it has a concentration of 0.05 percent tretinoin. The cream comes in two strengths: 0.05 percent and 0.1 percent (the strength used in the Michigan study). The gel has much lower concentrations -- 0.01 percent and 0.025 percent -- but because it penetrates the skin surface better it can be much more irritating even at these low doses.

For treating acne, dermatologists usually recommend using the highest dose of Retin-A that will be tolerated. But for reversing photo-aging, which is likely to require a lifelong regimen, the consensus is that the lowest-strength cream, 0.05 percent, is probably the most suitable.

The developer of Retin-A, Albert Kligman, thinks that concentrations as low as 0.01 percent may eventually be useful as an anti-aging cream.

Dermatologists recommend the following steps for using Retin-A, to minimize the irritation and scaling that can accompany its use:

Use Retin-A in the evenings only.

Wash the face first with a mild soap (Dove is good), pat dry and wait about 20 minutes for the skin to dry thoroughly.

Apply only a small amount of Retin-A in a very thin layer around the face, avoiding the eyelids, eyes and nostril flares.

The next morning, wash again with a mild soap to remove the Retin-A and apply a good moisturizer (such as Vaseline, Lubriderm or Nivea cream).

Use a high-protection sunscreen on your face every day; Retin-A peels away the outer layer of the epidermis and makes the skin extremely sensitive to the sun's rays.

If the skin gets very red, scaly or irritated, cut back use to every other night or even stop the medication for a few days, until the irritation improves.

Be patient; visible improvement may be as long as six months away.

Plan to use Retin-A for at least one or two years to reverse photo-aging, and then be prepared to maintain a twice-a-week routine for the rest of your life.