They are different drugs with similar function:

Retin-A is a natural retinoid (aka Retinoic Acid, Tretinoin) derived from Vitamin A. It activates all of the RAR type retinoid receptors to elicit it's effects on the skin (collagen production, collagen protection, possible DNA repair, clearing the pores, normalizing keratin production to keep pores clear, speeding up cell turn over, etc.)

Tazorac is a synthetic retinoid (meaning it is not derived from Vitamin A,) but it also activates RAR receptors (though no evidence that it might activate RXR's too, which tretinoin might.) Basically, scientists used what they figured out about how Retin-A works to design this drug, so that it is targeted to specific actions in the skin.

Differin and Tazorac have something in common - in that they are both synthetic retinoids. (i.e. They are not made from Vitamin A) but they activate retinoid receptors, so they are still classified as retinoids.

The difference is that they activate different sets of receptors, so they give slightly different responses in the skin. Differin is not a weaker form of Tazorac. It is more mild, and they are both retinoids, but they are distinct drugs.

Differin is one of the most mild retinoids you can use. I would say it is a good place to start. Since it's so mild, be aware that the trade off is that it doesn't work as great. It will work for most people, but usually takes longer to see full results.

Tazorac is probably the strongest retinoid you can use. It was developed to treat plaque psoriasis, and is now licensed to treat acne. It works very well, but the trade off is that it can be more irritating.

Here are some papers comparing the various Rx retinoids about the tolerance levels:

As you can see, Differin won the irritation test in all trials (in this journal,) but I believe that all of the Rx retinoids are great. It's just that some work better than others for different individuals. It usually requires some experimenting to see what works for you.

Differin vs. Tazorac

http://www.cutis.com/asp/archive/article.a...mp;FileType=abs

Cumulative Irritation Potential of Adapalene 0.1% Cream and Gel Compared With Tazarotene Cream 0.05% and 0.1%

Dosik JS, Homer K, Arsonnaud S

2005;75:289-293.

abstract

Despite the many beneficial effects of dermatologic applications, most of the current treatments for acne cause local irritation. The objective of this study was to compare the ability of the epidermis to tolerate adapalene 0.1% cream and gel and tazarotene cream in concentrations of 0.05% and 0.1%. A total of 30 subjects were enrolled in the study. The test products were applied under occlusive dressings at randomized sites on the upper back for approximately 24 hours, 4 times a week, and for 72 hours, once a week, for a period of 3 weeks. Skin reactions (erythema score plus other local reactions) at the product application sites were assessed 15 to 30 minutes after dressing removal. Twenty-six subjects completed the study. A total of 16 subjects discontinued use of 1 or more of the test products because of irritation scores reaching severe or greater; all but one of these discontinuations were at sites treated with the tazarotene products. The mean 21-day cumulative irritancy indices for adapalene 0.1% cream and gel were significantly lower (P.05) than those for tazarotene cream 0.05% and 0.1% and not notably higher than that of the negative control product.

Differin vs. Retin-A Micro

http://www.cutis.com/asp/archive/article.a...mp;FileType=abs

Cumulative Irritation Potential of Adapalene 0.1% Cream and Gel Compared With Tretinoin Microsphere 0.04% and 0.1%

Dosik JS, Homer K, Arsonnaud S

2005;75:238-243.

abstract

Despite the many beneficial effects of dermatologic applications, most of the current treatments for acne cause local irritation. The objective of this study was to compare the ability of the epidermis to tolerate adapalene 0.1% cream and gel and tretinoin microsphere in concentrations of 0.04% and 0.1%. A total of 31 subjects were enrolled in the study. The test products were applied under occlusive dressings on the upper back for approximately 24 hours, 4 times a week, and for 72 hours, once a week, for a period of 3 weeks. Skin reactions (erythema score plus other local reactions) at the product application sites were assessed 5 to 30 minutes after dressing removal. Twenty-six subjects completed the study. A total of 10 subjects discontinued use of 1 or more of the test products because of irritation scores reaching severe or greater; all of these discontinuations were at sites treated with the tretinoin products. The mean 21-day cumulative irritancy indices for adapalene 0.1% cream and gel were significantly lower (P,.01) than those for tretinoin microsphere 0.04% and 0.1% and not higher than that of the negative control product.

Differin vs. Tazorac & Retin-A

http://www.cutis.com/asp/archive/article.a...mp;FileType=abs

Cumulative Irritation Comparison of Adapalene Gel and Solution With 2 Tazarotene Gels and 3 Tretinoin Formulations

Greenspan A, Loesche C, Vendetti N, Georgeian K, Gilbert R, Poncet M, Baker MD, Soto P

Cutis. 2003;72:76-81.

abstract

Forty-two subjects with normal skin were enrolled in a single-center study to assess the cumulative irritancy potential of adapalene (Differin¯¿½ gel 0.1% and Differin solution 0.1%) compared with tazarotene (Tazorac¯¿½ gels 0.05% and 0.1%), tretinoin (Retin-A Micro¯¿½ gel 0.1%, Avita¯¿½ cream 0.025%, and Avita gel 0.025%), and white petrolatum (negative control). All test materials were applied randomly, under occlusion, to sites located on either side of the midline¯¿½the mid thoracic area of the subjects¯¿½ backs. All patches were applied daily, Monday through Friday, to the same sites, unless the degree of reaction to a test product or adhesive necessitated removal (grade 3).

Thirty-eight of the 42 subjects (90.5%) completed the study. Thirty-four of those 38 subjects (89.5%) had to discontinue using both tazarotene concentrations due to intolerance. Patch discontinuations for the remaining test materials were as follows: 7 subjects discontinued use of tretinoin microsphere gel 0.1%, 3 discontinued tretinoin cream 0.025%, 1 discontinued tretinoin gel 0.025%, and 1 discontinued adapalene gel 0.1%. None of the subjects discontinued use of the white petrolatum or the adapalene solution 0.1%. Adapalene gel and solution 0.1% were statistically (P<.01) less irritating than both tazarotene gels 0.1% and 0.05%, tretinoin microsphere gel 0.1%, and tretinoin gel 0.025%, and they were not statistically different from tretinoin gel 0.025%.

Concerning retinoid receptors and the differences between the drugs:

Differin activates RAR-beta and RAR-gamma, but it only activates RAR-alpha very weakly (to almost no effect.) However, as the data indicates, the two that Differin activate strongly are enough such that they reduce the symptoms of photoaging.

Tazarotene activates all three RAR's strongly.

Tretinoin activates all three RAR's strongly, and there is evidence it may indirectly (i.e. via other proteins) activate the RXR's too, but I can't find too much info on this.

There are some new retinoids in clinical trials now (for photoaging and acne... they are already used to treat certain skin cancers in aids patients, I think,) so maybe we can find a way to do combo-therapy to be sure all six are being activated