The keto flu is not an actual flu or illness for that matter. It’s a set of ketosis symptoms that may appear during the induction phase of a ketogenic diet when the body is adapting to changes in nutrient availability . These symptoms are also commonly called "carbohydrate withdrawal symptoms."
Keto flu symptoms include: headaches, “brain fogginess”, feeling lethargic, nausea, muscle cramps and irritability. It happens because your body is burning up what's remaining of its glycogen stores and because ketosis has a diuretic effect, meaning you’re losing a lot of electrolytes and water. That's also why you can reduce and eliminate most keto flu symptoms by staying hydrated.
With all this in mind, having uncomfortable symptoms early in your keto diet does not mean keto is not for you. It is just a pasing phase. You can expect keto flu symptoms to dissipate in a few days to 3 weeks on this diet. However, some people find that it takes them longer to get over the keto flu. That is more likely to happen if you are detoxing from a diet high in refined carbohydrates and sugar, so your keto flu is also resulting from carb withdrawal.
To fight the keto flu, stay well hydrated and include keto diet foods rich in electrolytes (sodium, magnesium, and potassium). If necessary, take supplements, especially magnesium or even exogenous ketones.