“Fasting puts you in nutritional ketosis as does ketogenic / very-low-carb dieting. But the metallic taste is usually a temporary and harmless keto-adaptation symptom. … The metallic taste is excess ketones. If your body is not used to using ketones, some will be excreted in the urine and saliva.”
(Copied off of google)
Yup. My teeth feels sticky, as if they are coated with something.
On the other hand you don’t need to buy anything to measure your ketones, you already know when you hit the sweet spot. if you journal your fasts start tracking this as well.