Amanda Seyfried is getting candid about her mental health.

The 30-year-old actress talks about dealing with OCD and anxiety in a new interview, and advocates for mental health to be taken as "seriously" as other diseases.

"I'm on Lexapro, and I’ll never get off of it," Seyfried tells the November issue of Allure, referring to the antidepressant used to also treat anxiety. "I've been on it since I was 19, so 11 years. I'm on the lowest dose. I don't see the point of getting off of it. Whether it's placebo or not, I don't want to risk it."

WATCH: Amanda Seyfried Really Wants a Child: 'I Keep Feeling Like My Eggs Are Dying Off'

"And what are you fighting against? Just the stigma of using a tool?" she adds. "A mental illness is a thing that people cast in a different category [from other illnesses], but I don't think it is. It should be taken as seriously as anything else. You don't see the mental illness: It's not a mass; it's not a cyst. But it's there. Why do you need to prove it? If you can treat it, you treat it."

The Mamma Mia! actress shares her own experience of getting diagnosed.

"I had pretty bad health anxiety that came from the OCD and thought I had a tumor in my brain," she recalls. "I had an MRI, and the neurologist referred me to a psychiatrist. As I get older, the compulsive thoughts and fears have diminished a lot. Knowing that a lot of my fears are not reality-based really helps."

Seyfried, who got engaged last month to her Last Word co-star Thomas Sadoski, also admits to feeling intensely insecure at times.

"Sometimes I feel I know the world so well, but then...it's so debilitating," she explains. "You're like, 'What am I doing here? No one wants to see me. Why are you taking my picture?' It's stupid, it's irrational, and it's not all about me, but I make it about me because I'm insecure."

WATCH: Amanda Seyfried Says She Was Paid Only 10% of What Her Male Co-Star Made

Last May, Seyfried also opened up about seeking therapy after watching her infamous drunk appearance on The Late Show With David Letterman in 2012. The actress admitted at the time to drinking whiskey backstage to help calm her nerves.

"It made it fun for me, but then I watched it and was like, 'That is not what I want to promote about myself,'" she told Vogue. "I have a lot of anxiety that I've been struggling with my whole life. So I have been working through it. I'm terrified, but this is exactly what I wanted."

Watch the video below for more on Seyfried and 40-year-old Sadoski's relationship.