A father and son suffered internal organ injuries and were hospitalized on Monday after drinking hot chocolate at a McDonald’s restaurant in Southwest China's Chongqing, where an employee mistakenly added disinfectant instead of hot water to the drink.

Feng Lili, 46, took his four-year-old son to a drive-through McDonald's near the Yuan Yang subway station in the Yubei District Monday at noon. They ordered a kid's meal, which included a hot chocolate.

The son took a sip first and refused to drink it anymore. Feng thought maybe the drink was too hot and asked the son to wait until it cooled down. After the son finished the meal, Feng found the drink almost untouched and decided to drink it himself, according to Chongqing Evening News.

"I had a gulp of the drink and it tasted a little bit sour," Feng was quoted as saying. "I thought it might be Sprite in it. But when I took a second sip, I found something was wrong."

After about 10 seconds, Feng felt burns on his tongue and throat. He returned to the restaurant and took what was left of the drink to the counter and told employees about his sickness.

McDonald's staff admitted that a colleague mistakenly added disinfectant instead of hot water to Feng's drink while the store was doing its routine disinfection of kitchen utensils.

The store explained that the hot water bottle was full of disinfectant at the time, but the staffer thought hot water in it and poured it into Feng's drink.

The father and son then went to Daping Hospital with the McDonald's workers.

The doctor diagnosed that both were poisoned by the disinfectant, which injured their alimentary canals.

According to the Chongqing Liangjiang Food and Drug Administration, the son has recovered and the father is still in the hospital under medical observation.

"The McDonald's staff did mistakenly add 250 grams of disinfectant (deliquated Cyanuric acid sodium chloride) into Feng's hot chocolate (80 ml taken)," according to the statement by the food and drug authority.

The investigation on this incident is still underway.

"McDonald's has been dedicated to providing our customers with safe and high-quality food and we sincerely apologize for the misconduct at our store," McDonald's said in a press statement released to China Daily Tuesday afternoon.

"We will take active action to solve the problem with the customer and strengthen our management and staff training to prevent incidents like this from happening again."

Feng declined to comment on the incident.

Deng Rui contributed to the story.