Australian actress Margot Robbie has stunned beauty product devotees by revealing she swaps lip balm for nipple cream in order to keep her lips hydrated.

Speaking to Elle UK, the 26-year-old said she had a “conspiracy theory” about regular lip care products that led her to use the unconventional method.

“I use Bepanthen, which is actually a nipple cream for breastfeeding mothers or diaper rash cream for babies,” Robbie said.

“I have a conspiracy theory that lip balms actually have additives in them to dry your lips out so you keep buying them.

“But because Bepanthen is just a cream for dry skin, it works. It’s what I’ve used my whole life.”

While her methods might be unconventional, Robbie might actually have a point, according to Sydney-based cosmetic dermatologist Dr Natasha Cook of the Darlinghurst Dermatology Skin and Laser Clinic.

Dr Cook said nipple creams like Bepanthen are designed to “rebuild the barrier of the skin” on the nipple, as babies can create “lacerations” when breastfeeding, and thus it may do a better job than most lip balms.

“Some of the traditional lip balms are more cosmetic than they are rejuvenating of the skin,” Dr Cook told The New Daily.

“Sometimes things that are petroleum-based absorb water out of the lips and create a reliance. It makes [the lips] feel drier so you keep buying the product.”

Margot Robbie’s perfect pout is partly due to a cream used by new mothers – and it probably works better than most lip balms. Photo: Instagram

Dr Cook said other varieties of lip “waxes” or “ointments”, like Lucas’ Papaw ointment, are often similarly ineffective, while Vaseline “just sits on the surface”.

If you really want to repair and hydrate your lips, Dr Cook advises lanolin, derived from a wax secreted by woolly animals like sheep.

It might sounds gross, but Dr Cook says lanolin is the “closest thing to natural sebum”, which is the body’s natural skin and hair lubricant.

“Lanolin sinks in and heals the epidermal barrier and mimics the actions of natural sebums – it prevents water loss and repairs any damage,” Dr Cook explained.

Associate Professor Samuel Zagarella, also a dermatologist, said dry lips can be caused by dry weather and low humidity, and occasionally some skin diseases or medications can play a role.

He recommends olive oils or other vegetable oils for hydration.

Speaking to Real Simple magazine, New York-based dermatologist and director of cosmetic and clinical research at Mount Sinai Medical Centre Joshua Zeichner said some lip balms make the skin “lazy”.

“These formulas create an artificial barrier to lock in moisture, but when overused, can cause lips to become lazy because the skin doesn’t have to work to maintain its own hydration level.”