Cox told TV host Jimmy Kimmel she remembered very little about the hit sitcom in which she played Monica for a decade.
"People love the show so much, I decided to binge watch Friends... it's really good!" she said via video link.
"I don't remember even being on the show, I have such a bad memory."
She added: "I remember obviously loving everybody there and having fun and I remember certain times of my life I was there, but I don't remember episodes."
The hugely successful show notched up 236 instalments and has enjoyed a recent resurgence among a new generation on Netflix.
Cox was chatting with Kimmel on his YouTube Quarantine Minilogue, which he began after his late-night talk show was taken off air as a result of the pandemic.
The actress said she had bought the entire Friends box set on Amazon Prime after it was taken off Netflix in the US in anticipation of the HBO Max streaming service launching in May.
All 10 series are still available on Netflix in the UK and episodes also run on Channel 5 and Comedy Central.
Due to her foggy recollection, Cox said she gets stressed out when people ask her to answer questions about Friends.
That was a red rag to a bull for Kimmel, who decided to test her knowledge in a head-to-head with his cousin, a huge fan of the show.
Despite the questions all being Monica-related, Cox lost 5-0.
Questions she did not know the answers to included who Monica shared her first kiss with, and who peed on her leg in season four episode The One With The Jellyfish.