Lovato, who has suffered with eating disorder, took to Twitter to criticise Gaga for glorifying bulimia.
"Sad... As if we didn't have enough people glamourising eating disorders already. Bottom line, it's not 'cool' or 'artsy' at all," the 21-year-old singer said.
Gag is seen sticking her finger down her throat and throwing up at one time during her concert but while Gaga's fans may have found it another of singer's attempt at pushing the envelop, Lovato and many other Twitter users have criticised the image.
Lovato has had her own demons to fight in the past. The singer entered rehab in 2010 to seek treatment for physical and emotional issues.