“Pre-exposure prophylaxis: Loading dose: 800mg of hydroxychloroquine salt (620mg base) taken at 0-hours, then 400mg (310mg base) taken at 6-hours, 24-hours, and 48-hours. Ongoing treatment: 400mg hydroxychloroquine salt (310mg base) taken once weekly.”
Ref the following link: https://keleefitness.com/covid-19-prophylaxis-in-healthcare-workers/
That’s current practice. If I were being regularly exposed, I’d want the hammer in my dose.
The lower limit isn’t known, nor are its more subtle prophylactic effects at lower dosages (if any). A lot of lab animals are going to find out, I’d guess.
“Tonic water contains no more than 83 mg of quinine per liter—a much lower concentration than the 500 to 1,000 mg in the therapeutic dose of quinine tablets.”
https://www.health.harvard.edu/bone-and-muscle-health/will-tonic-water-prevent-nighttime-leg-cramps
So you’d need three liters to get over 240 mg of quinine.