Zellweger's changed appearance during an Hollywood event had sparked plastic surgery rumours which the actress attributed to a relaxed lifestyle.
Speaking about the speculation surrounding Zellweger's face, Witherspoon told the Hollywood Reporter that such attention to female star's appearance bothers her "immensely".
"It's horrible. It's cruel and rude and disrespectful, and I can go on and on and on. It bothers me immensely," Witherspoon said of the critics.
"I know this is so Pollyanna of me, but why - and it's particularly women - why do they have to tear women down? And why do we have to tear other women down to build another woman up? It drives me crazy.
Zellweger addressed the rumours surrounding her changed appearance by saying that a "fulfilling life" may be behind her relaxed looks.
"I'm glad folks think I look different! I'm living a different, happy, more fulfilling life, and I'm thrilled that perhaps it shows. People don't know me in my 40s. People don't know me healthy for a while. Perhaps I look different. Who doesn't as they get older?! Ha. But I am different. I'm happy," she had told People magazine.