I resumed a ketogenic diet last month because I started having issues with carb crashes and brain fog again. The last time I tried the diet I gave up after approximately 3 weeks due to lethargy and weakness. I hit the 3 week mark at the beginning of this week and encountered the same symptoms. There is an excellent subreddit on keto dieting and several people noted that this was normal (even after 3 weeks) and usually due to lack of fluid and electrolyte management. I found that after increasing fluid and electrolyte intake I felt much better. I’m now enjoying the cognitive benefits and constant energy levels this diet is known to provide.