White smoke out of exhaust

[djmarch]

I was wondering why it started to use about a 1/2 galllon of water every 5-600 miles. Changed the thermostat a few months ago thinking that was the problem, but now getting considerable amount of white/brown smoke out the exhaust. No water in the oil. I'm hopeing its just a head gasket and not a cracked head or block. Only has 72,000 miles on the engine. I'm thinking about dumping a can of Barrs head gasket sealer in it and see what that does to get me through to spring, but I doubt it will.

How much damage can these cans of over the counter head gasket sealers cost in the long run ? And why would I have a bad head gasket with only 72k miles ?