Amber Tamblyn once drank some of her friend Blake Lively's breast milk.

The pair have remained close pals since starring in 2005 movie The Sisterhood of the Traveling Pants, along with America Ferrera and Alexis Bledel.

And as she appeared on U.S. chat show Watch What Happens Live on Wednesday night (17May17), Amber admitted their bond is so strong that she has even tasted Blake's lactations.

Her admission came after the show's host Andy Cohen asked Amber about her husband David Cross' recent revelation that he enjoys the taste of his wife's breast milk.

At that point, the actress offered Andy the chance to take a sip of her milk from a small bottle, at which point the presenter said: "Am I drinking it here? Am I literally going to do this, is this really happening?"

As Amber and the audience egged him on with a chant of "chug, chug, chug", openly gay Andy tried the milk, before saying, "It's very sweet, it's sweet. And it's good with a tequila chaser. Wow, I'm schvitzing. Can I have more? Just kidding. That's the closest to a breast I've ever been. Wow."

Members of the crowd then started teasing Andy that he was now an official member of The Sisterhood, with Amber sharing: "That's actually literally true because I've drank Blake Lively's breast milk, and so now technically if that went through me, you've technically had Blake's and my breast milk together."

Making another quip, Andy laughed: "So does that mean I've done it with Ryan Reynolds? That would make it all better."

Blake and Ryan have been married since 2012 and have two daughters together, while Amber and David also married in 2012, and welcomed their first child together, daughter Marlow, earlier this year (17).

For the first time since the birth, Amber chose to cut loose a little when celebrating turning 34 on 14 May. Amber drank some bourbon whiskey at a party and the birthday booze changed the flavour of her breastmilk, which David confessed he has tried multiple times since becoming a dad.

"It was amazing!" he told New York Daily News gossip column Confidential. "I had some (of her breastmilk) very early on; it was very sweet. And then last night (during Amber's birthday party), I had the tiniest little bit. It was not sweet; it was very decadently creamy, fatty... It tasted like a dessert you would pay $50 for."