Combine the sugar, 1 cup of water, and juniper berries in a small saucepan over medium-low heat. Simmer gently for 20 minutes. Allow to rest, uncovered, for 4 hours, then strain through a fine-mesh sieve.

Combine the lime juice and juniper syrup in a cocktail shaker, then divide between 4 ice-filled highball glasses. Top with tonic water and serve garnished with sliced lime.