In a word: purity.
A concentrated form of vitamin A, retinoids increase cell turnover and boost collagen production, leaving skin radiant and smooth. Products you can find in beauty stores like Sephora or Space NK are over-the-counter (OTC), and will contain formulations like retinol and retinyl palmitate. These are not pure forms of retinoic acid—rather, they convert to retinoic acid once they’ve been absorbed by your skin.
Prescription formulations require a visit to your dermatologist and pharmacist because you’re dealing with something a bit more powerful—pure retinoic acid, (or a close compound called trans-retinoic acid)—which can be up to 20 times stronger than what you can find at a store.
While this purity may provide a quicker (or a stronger) benefit, the strength of the prescription formulations may irritate skin. If you’re noticing any unusual redness or irritation when using a retinoid product of any formulation, talk to your dermatologist.