Danger of using expired tretenoin acid/Retin–A?

A friend found an unopened tube of Retin–A in a travel bag that was
stashed in a cool, dark place. She wants to know if it's two years
past the expiration date, is it safe to use. I tried to google the
info for her, but couldn't find anything. Is it ineffective now?
Irritating? OK to use?
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© Anya {{{*_*}}}
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