Usually what I buy. In any case, it's one or twice a year. A little heartburn never killed anyone.
My other advice with port - and I write as someone who loves the stuff, especially the really good stuff - is hydration. Make sure you have plenty of water to hand, and drink it while you are imbibing your port.
This is a drink that leaves - or can leave - a nasty calling card - port hangovers can be quite unpleasant.