Could be an allergic reation, but more than likely what I have experience.
If you only take hydro occassionally, when you take it-your body secretes more facial "oil" than usual. This is only during a time when a persons body is getting used to the hydro.
I didn't get bad acne but did ask my MD about having an oily face while on hydro?? He said very common.
Also, about acne; my dermatologist told me to only wash very gently twice daily with Cetaphil and Cetaphil moisturizer. It worked great. After about two weeks, I was pretty clear. You also may want to ask your Dr. (PCP or dermatologist) about "TRIAZ Pads". It works great for minor acne. It reduces swelling and redness.
Sorry to banter but I know how psychologically devaststing mild/moderacte acne can be.
My doctor reminded me that washing 3 or 4 times a day did nothing but make the acne worse (and red). He said it is a bacteria and over washing definately does not help in any way.
KS
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The greatest trick the devil ever pulled was to convince the world he never existed.