From Dr Lee's site
RETINOIC ACID
"Topical all-trans-retinoic acid (aka tretinoin or retinoic acid or Retin-A) alone and in combination with minoxidil have been tested for the promotion of hair growth in patients with male pattern baldness.
There are postulated to be two mechanisms by which retinoic acid promotes hair growth in combination with minoxidil. Experimental evidence suggests that retinoic acid itself may play a significant role in hair growth and, because retinoic acid has been shown to increase percutaneous absorption (2,3), it is probable that the increased absorption caused by retinoic acid increases the amount of minoxidil reaching the hair follicle cells.
For the treatment of male pattern baldness, it is recommended that 1ml of 5% or higher concentration minoxidil topical solution be applied to the affected area twice a day. In the morning, 1ml 5% minoxidil solution alone should be applied to the areas of the scalp with thinning hair. The nighttime application should contain 5% or higher minoxidil concentration with 0.025% retinoic acid.
With the initial use of retinoic acid on the scalp, a small percentage of patients experience an apparent increase in hair loss, which was attributed to shedding of hairs in the telogen phase. The hair loss or shedding would have occurred without treatment and the same hair shaft replaces itself in about 3 months. But because the retinoic acid decreases the chemical bonding between the skin cells, hair that is in the telogen (resting) phase comes out more readily after using retinoic acid.
The proprietary preparation is commonly known as Retin-A, manufactured by Ortho Pharmaceutical. Pure retinoic acid is intensely yellow, so that unadulterated compounds of retinoic acid will have a bright yellowish cast. Retinoic acid has been used for more than 25 years in the treatment of acne, and tests using retinoic acid alone to stimulate hair growth have been conducted since as early as January, 1977 when a British patent application was filed and December 1977 when a German patent was filed.
More recently, retinoic acid has been highly touted to reverse the aging of the skin, primarily due to solar exposure. In fact, U-V light is considered to be the chief contributing factor to aging skin. Therefore, unnecessary exposure to the sun is discouraged while using retinoic acid. Basically, the retinoic acid acts like a chemical peel. Erythema (redness), flaking, peeling and mild irritation frequently occurs with the use of retinoic acid, but is usually mild and limited to 7-10 days. As with minoxidil, the effects of retinoic acid on the skin are dose related. The .025% concentration of retinoic acid has become the standardized concentration among dermatologists. True contact allergy to topical retinoic acid is rarely encountered (1). The reaction in the skin is to induce protein synthesis as well as to increase cell turnover. Since retinoic acid is decomposed by strong light, it should only be applied at night.
(1)Nordquist BC, Mehn K: Allergic contact dermatitis to retinoic acid. Contact Dermatitis 3:55-56, 1977
(2)Zbinden G: Pharmacology if vitamin A acid (13-cis, all-trans-retinoic acid). Acta Derm Venereol (Stockh) 55:21-24, 1975
(3)Ferry JJ, Forbes KK, LanderLugt JT and Szpunar GJ: Influence of tretinoin on the percutaneous absorption of minoxidil from an aqueous topical solution. Clin Pharmacol Ther "