I Have Both Retin-A and Clindamycin Phosphate Gel, How Do I Use Them Both Properly?

Retin-A- .04 percent. My doctor said something after school to put it on but I'm reading online that it's best to be used at night Clindamycin Phosphate gel - Now I'm not sure if this was for my face or back but Im guessing my back because I had a lot of back acne. My doctor looked at my face and said I have white headds and black heads but not those big red pimples which is why Im guessing i shouldnt use the clindamycin on my face. But on my back are the big red pimples.

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