Why does vitamin c make me depressed? its a 250mg vitamin c but whenever i take vitamin c i feel mentally lethargic and slowed thinking. whys this?

Unexpected effect: Definitely not expected in this context. With limited information available best would be to stop taking vitamin C for a week or so in trying to answer a simple question if they are related. In case it proves to be related next will be to question with your physician the need of continuing taking it in the context. Ultimately if still needed would pick either a different brand or strengtg.