You can use vodka or whiskey, if you prefer, in lieu of the rum. Note that the higher the proof (ABV), the better the extraction of flavors, so feel free to use any of those spirits with a higher alcohol level. I made the gentian root optional but it adds a husky bitterness that I find really makes the bitters, better. You can find it at herbalists, natural foods stores, or online at Penn Herb, Mountain Rose Herbs, and Etsy. You can also play around with the spices. A bit of cinnamon, a few cloves, or perhaps a bit of star anise could be added. Be sure to filter the mixture through a coffee filter, rather than cheesecloth, which will insure that you won't have little bits of spices in the finished bitters, which will make it cloudy.