All glycerin is is the same chemical, but if your bottle says "for external use only," it may not be as chemically pure as you'd like. Drug store glycerin is often fine for consumption (it's where I've bought it), but check the label.

Ideally I believe you want vegetable glycerin, I bought some at Whole Foods; it was in the cosmetics department but the bottle has directions for cooking use. What does the label say?