Tim Allen insists Roseanne Barr was once the most ''tolerant woman'' he knew.

The 56-year-old TV star has known Roseanne for many years and he's admitted to being shocked by the offensive tweet she posted on Twitter about Valerie Jarrett, a senior adviser to former President Barack Obama, earlier this year.

He confessed: ''I go way back with Rosie and that's not the Rosie I know. She was the most diverse and tolerant woman I've ever known for a long time. Whatever got in her head, isn't the Roseanne I know.''

Roseanne, 65, was fired by ABC and her eponymous sitcom was cancelled by the American TV network after she likened Jarrett to an ape.

Tim feels that in the current political climate, comedians and other public figures are having to be more cautious than ever with their comments.

He explained: ''I'm a veteran comedian for 38 years and I've never seen it, like Lenny Bruce said at the Purple Onion, 'We've gone backwards.' There are things you can't say. There are things you shouldn't say.

''Who makes up these rules? And as a stand-up comic, it's a dangerous position to be in because I like pushing buttons. It's unfortunate.''

Earlier this week, controversial actor Charlie Sheen said he could ''relate'' to Roseanne.

Charlie - who was previously fired from 'Two and a Half Men' after he criticised the show's creator, Chuck Lorre - explained: ''I can relate to that tone of absolute despair because it's not just about herself, it's about the people that she knows she affected as well.

''What I hear in her voice, trying not to focus on the words but the emotion, is I hear the frustration, pain, there's such a sadness there.''

The Hollywood veteran also thinks it's ''a little bit sad'' that Roseanne's tweet has destroyed her entire legacy.

He added: ''The thing that is a little bit sad is that when someone does melt, everything good they have ever done that has entertained millions of people is always forgotten.''