Why does ‘Tonic Water’ glow in the dark?

Things glowing in the dark can remind you of your childhood days when you used to experiment with your friends. This may seem like a fun thing to do, but it’s actually true! If you were to look at a gin and tonic (or vodka and tonic, or anything with tonic, for that matter) under ultraviolet light, you might be surprised to see it glow and you might naturally wonder what produces the eerie effect.

The answer is surprisingly simple. Tonic water actually contains the chemical quinine, which fluoresces under certain lights and makes tonic water glow. According to Scientific American, “In general, something fluoresces because it has absorbed light energy, which makes it excited, and then it releases (or emits) light as it returns to its normal, unexcited state.” Quinine absorbs ultraviolet light and then re-emits it as visible light, meaning it appears to glow when other materials remain dark.

But why does tonic water contain quinine in the first place? Because it was once used to treat malaria. The quinine powder that British colonialists in India had to take to prevent malaria was so bitter that they began mixing it with sugar and soda water, which, in turn, they mixed with gin. Voila, a classic British cocktail came into being.

While it may have been a bit weird to read about how your go-to drink or cocktail mixer actually glows in the dark, it turns out that the amount of quinine in tonic water isn't anything to be concerned about. Originally used to help treat malaria patients, quinine is safe to consume in small doses, such as the amount found in tonic water. In fact, the amount in tonic water is diluted enough that you shouldn't feel any type of side effects from the beverage.