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I read on https://www.medscape.com/viewarticle/726464_6:
Owing to potential photosensitivity, topical retinoids are best applied at night and patients should not expose themselves to excessive UV light.
For how long does the application of topical retinoids increase photosensitivity?
I searched for "For how long does the application of topical retinoids increase photosensitivity" without success.
They increase UV sensitivity for as long as you use them, so sunscreen is recommended if doing any activity outside in the sunlight.
Retinoids simply make your skin more sensitive because it stimulates renewal of skin cells and these cells are thinner and more photosensitive, allowing more UV to penetrate and cause harm.
Also, the retinoids themselves are sensitive to UV radiation so the sunlight would reduce the effectiveness.
There are studies going back to the eighties about retinol side effects which cite these two issues.
https://www.skincancer.org/blog/when-beauty-products-cause-sun-sensitivity/
