The 58-year-old man was at his local bar when he complained to his friend, a bartender at the bar, that the wine he just served him was of bad quality, The Local.es reported Tuesday.
The bartender, whose name was not reported, allegedly decided to get back at him by serving him sink unblocker instead of wine as a joke, the news site said.
Once the alleged victim drank the acid, he began to shout and fell to the floor in convulsions, The Local.es said.
The man was transported to Granada Hospital where he is in critical condition. Police were still searching for the bartender, who initially said he didn't realize the sink unblocker was toxic.