Doxycycline twice daily, aqua glycolic facial cleanser, sodium sulfacetamide 10% and sulfur 5%.My face dries from time to time;i would like to know why.

Modify regimen. Sulfacetamide/sulfur often causes dryness, particularly during the winter. I would first ask your dermatologist to substitute something milder. You can also use a light moisturizer as needed if you wish, but choose one that says "non-comedogenic" on the label, meaning that it won't plug up your oil glands and make your acne worse.