I recently discovered Portland Syrups and their Cola flavor. Now this is not zero calorie, but fairly low for the amount you need. You mix it with plain seltzer for a somewhat cola flavored drink. The strength varies by how much of it you use. For myself, no more than a teaspoon for an 8 ounce glass. They have other flavors as well, such as Ginger Ale and Root Beer. I cannot drink the various zero cal beverages as they taste very nasty to me, but to each their own.