What’s the correct character for the act of showing your teeth? The word to show your teeth in Chinese is 呲牙 or 龇牙? I have seen 龇牙 in dictionaries but 呲牙 in like Wechat and other places. So do they have any differences or are they the same?
They are same meaning, just has different written words in some ancient books.
Check the Chinese sentences below:  


齜牙咧嘴 教育部重編國語辭典修訂本  
呲牙裂嘴 教育部重編國語辭典修訂本