Friends fear the super skinny star is obessed with losing weight.

A gaunt and pale Courtney shocked fans at a surprise gig for her 43rd birthday in London, having lost more than 23kg and at least three dress sizes in what she calls an "anthropological experiment".

"I was at a charity event and my friend Sting said, 'Why don't you do a body transformation?' I was about to start on a new album and was feeling experimental. It made me wonder what would happen if I got really skinny," she says.

However, friends believe the singer/actor has taken her dieting obsession way too far.

More alarmingly, Courtney admits to passing out after becoming dizzy from fluid loss after taking diuretics. She also does daily workout sessions, including Pilates and running.

"It's like kicking drugs," she says of her extreme measures.