Bring the water to a boil.
Add the shiso leaves. Turn the heat down and simmer for 5 min.
Strain through a sieve into another pot.
Add sugar. Mix well to dissolve. Simmer on low heat for 5 min.
Turn the heat off. Add lemon juice. The color will turn to vibrant red as you mix.
Let the mixture cool down.
Dilute with regular, sparkling, or soda water to taste and serve over ice.
Store the unused syrup in the refrigerator, or keep it in the freezer.
