Dean McDermott -- Milking Rehab For All It's Worth

Dean McDermott's been hitting the bottle in rehab ... but this beverage won't derail his treatment because it looks like it's all part of his plan to get healthy.

Dean left his rehab facility over the weekend for an outing -- with other patients -- and carried his own refreshment ... a healthy bottle of Kefir -- a vitamin-rich fermented milk drink.

McDermott checked himself into a treatment program in L.A. last week for "health and personal reasons" ... and apologized for "the pain" he caused his family -- an apparent reference to reports he cheated on Tori Spelling.