En el ácido

En el ácido literally means 'on acid', as in 'on LSD'. It's an expression that has nothing to do with the actual consumption of illegal drugs. En el ácido is used to refer to an extremely upset or perturbed person, usually by an undesirable  situation. The person en el ácido  often behaves as if he or she were hallucinating with the help of LSD: frantic looking eyes. sweating, not-making much sense, etc.

Example:

Leticia: Creo que está en el ácido. 
Juanjo: ¡Puta madre!, y cómo no si le dieron bajilla con la laptop donde tenía todos los documentos de su chamba. 

Leticia: I believe he is utterly upset.
Juanjo: Geeez, I bet he is, he's laptop was stolen with all the documents from work.