If i shared a straw with someone who has had a recent cold sore outbreak, can i take valtrex (valacyclovir) as prophylaxis? if so, what dosage would be appropriate?

Not needed: Nearly everyone has been infected with the HSVI associated with cold sores and thus being exposed to a cold sore would not increase your risk in getting cold sores which only affect a relatively small number of people.

500 twice a day: If you don't have an outbreak but are wanting to use it for prophylaxis, then the dose would be 500mg every 12 hours for 3 days.