A new mobile phone game on WeChat, a social network app in China, is proving so popular that it's causing hand injuries for many Chinese users.

The game, which was recently introduced to WeChat's 400 million subscribers, allows users to compete online with their friends in aircraft battles.

Within two hours after the release of the game on Aug 5, there were more than 180 million downloads.

But Zou Cheng, an orthopedic doctor at Hangzhou First People's Hospital in Zhejiang province, said he has received at least five patients this week with symptoms of tenosynovitis, an inflammation of tendons, that he said was caused by an overzealousness with the game, according to a report by Qianjiang Evening News.

Most of them, the doctor said, are women, and the site of injury usually occurs on the thumb.