IIRC modern tonic water doesn’t have a high-enough quantity of quinine to be medically-useful unless you drink rather a lot of it. I suspect that you’d die of alcohol poisoning prior to curing malaria.

googles

https://en.wikipedia.org/wiki/Tonic_water

In the United States, the US Food and Drug Administration (FDA) limits the quinine content in tonic water to 83 ppm[8] (83mg per liter), while the daily therapeutic dose of quinine is in the range of 500–1000mg,[9] and 10mg/kg every eight hours for effective malaria prevention (2100mg daily for a 70-kilogram (150 lb) adult).[10]

Okay, so if your tonic water is sitting at the maximum legal quinine level and you’re that 150lb adult, you’re drinking 25 liters – 6.6 gallons – of tonic water a day, which…is probably going to give you water poisoning, much less alcohol poisoning.

https://cocktail-society.com/recipes/gin-and-tonic-ratio/

The gin-to-tonic ratio is one of the biggest questions when it comes to the classic Gin and Tonic recipe. You could go bold with a 1:1 ratio or opt for less boozy options like 1:2 or 1:3. We explain what ratio of gin to tonic is best for whom.

To make it short and sweet, the best ratio is 1:3 - one part gin to three parts tonic water. That offers the best of both worlds: Enough gin to highlight the botanical ingredients and enough bittersweet tonic water to balance alcoholic notes and make the drink super refreshing.

Assuming the optimistic 1:3 ratio there, that’s 2.2 gallons of gin a day.

https://www.arkbh.com/alcohol/types/liquor/gin/alcohol-content/

Gin must have a minimum 40 percent ABV (alcohol by volume) to legally be sold as gin

So at least 0.88 gallons of pure ethanol a day.

The median lethal dose (LD50) for Ethanol is 7060 mg/kg.

So for our 150 lb adult, LD50 is 494g of ethanol.

https://en.wikipedia.org/wiki/Ethanol

Density: 0.78945 g/cm³

One gallon = 3785.411784 cm³. Our human is drinking 3331 cm³ of ethanol, or about 2,630 grams. That’s about 5.3 times the LD50. He can’t go throw it up, either, or he’d lose quinine. I’m not gonna look up the rate at which he could dump the alcohol from his system, if he tried spreading the doses out evenly, but my guess is that he’s probably going to be taken down by alcohol poisoning before the malaria is taken down by the quinine.

The amount of quinine in modern tonic water is woefully insufficient for that.

One of my favorite drinks. But I live in Colorado, so I will need to find another excuse. So, er, it’s five o clock somewhere, am I right?