Google "Standards of Medical Fitness" Army Regulation 40-501
2–28. Skin and cellular tissues
a. Current diseases of sebaceous glands to include severe acne (706.1), if extensive involvement of the neck, shoulders, chest, or back is present or would be aggravated by or interfere with the proper wearing of military equipment, do not meet the standard. Applicants under treatment with systemic retinoids, including, but not limited to isotretinoin (Accutane(r)) are disqualified until 8 (eight) weeks after completion of therapy.
This is the start point. After that, everything is on a case by case basis. My son was on it and was told by his liaison to be off it at least 2 months before he had his physical.