What is the difference between isotrex topical gel and retin a (tretinoin)? Asked my derm for retin a (tretinoin) and he prescribed me isotrex topical gel instead

Acne. Apparently your doctor thought that your acne is more severe than you .Both drugs are retinoids but isotrex is a bit stronger.
(Iso)tretinoin. Retin-a is tretinoin (available in states as http://www.Drugs.Com/retin-a.Html) whereas isotrex (appears to be uk brand http://www.Netdoctor.Co.Uk/skin-and-hair/medicines/isotrex-gel.Html) is isotretinoin. When taken orally as a pill (don't eat cream/gel/lotion!), Isotretinoin first gained fame as Accutane (no longer sold in states under that brand).