Is it safe to drink acid?

Never drink or even touch a strong acid or base.

That said, people drink mild acids every day.

Some acids you probably have drunk already, e. g. citric acid is in fruit juices and soft drinks. Acetic acid is the main component (5-10%) in vinegar.

Stronger and more concentrated acid solutions, pH below 3, can be harmful or even toxic. A lot of acids are toxic anyhow!