You should send out a followup letter now, but five moinths is not a terribly long wait. There are many margazine that require at least six months to respond, and some few take nearly a year. In really extreme cases, I've waited almost three years.
I sold one story that had been out so long that when the acceptance letter came I couldn't even remember writing a story with the title listed in the letter. I had to look through my records to make sure there hadn't been a mistake.
There are also many magazines that only read and respond during certain months, so the wait at these can be very long.
It is time to send followup letters, but not time to write them off. There's nothing at all unusual about a magazine taking longer than five months to respond. Sometime smuch, much longer.
As a writer, it's best to spend all your time working on new stories, and pretty much none of your time thinking about stories you've already submitted.