Dakota Johnson says her menstrual cycle is "ruining her life".

Dakota Johnson

Dakota Johnson

The 'Fifty Shades Freed' star - who is dating Coldplay frontman Chris Martin - has candidly spoken about period struggles and how she finds it "really traumatic" when her boobs swell to "eight times bigger" than normal and other parts of her body and brain are affected by her hormones.

Asked what the one thing she has any unanswered questions about is when it comes to beauty, Dakota replied: "If I'm honest, my hormonal changes during my menstrual cycle are ruining my life.

"Every month. It's unbelievable. It's really f***ing amazing. I can't get a grip on it. I'm like, what?

"Every time, I'm totally scandalised about what happens to my body and my brain.

"My boobs are like eight times the size they normally are.

"It's really a traumatic thing, and it happens every month. I just can't get used to it."

The 29-year-old star works with her boyfriend's charity Global Citizen and has been learning about reproduction as an advocate for the organisation.

The 'Suspiria' actress has admitted that she is finding it "very tricky" to know which form of contraception is the healthiest and safest to use to prevent pregnancy.

She added to InStyle magazine: "It's really, really insane. It's also very tricky to figure out what type of birth control you're meant to be on, and if it's even healthy.

"This is something that, again, I'm advocating with Global Citizen: Reproductive education.

"What it means to have a woman body, and how do you honour it the right way?

"How do you not jam chemicals into your brain in order to feel good?

"It's a remarkable thing. I really would like to understand and be able to manage things a little better, know what's happening in my body, and know what I'm putting into it."