Almost 80% of people in the UK know at least two friends who have experienced mental distress, research suggests.
The Mental Health Foundation research also revealed many people did not want to admit their problems to friends for fear of what they would think.
Of these half said it was because they were ashamed.
The survey, of people with mental health problems and those who have supported mentally ill friends, marks Mental Health Action Week.
|People who feel mentally unwell might find it hard to discuss how they're feeling but friends can provide support
Dr Andrew McCulloch
Mental Health Foundation
A total of 49% of those who responded said they did not feel able to talk to their friends about their mental health problem.
Six out of ten people with mental health problems reported that when their friends did find out, they were concerned and 47% offered support.
Two in three people said their friend's mental health problem did not put their friendship under strain - almost half (41%) said it actually made their friendship stronger.
A total of 62% of people with mental health problems said it helped to have friends around and 41% revealed they received more help from their friends than their GP or own family.
Almost half of the respondents who knew a friend with a mental health problem felt that they did not know enough about mental health to give advice and a further 48% said better information would have helped them to support their friends.
Nearly three quarters of people admitted feeling frustrated because there was no simple solution to their friend's mental health problem.
Dr Andrew McCulloch, chief executive of the Mental Health Foundation, said: "Friendships are very important for good mental well-being, yet people can feel ashamed or embarrassed to tell their friends about how they feel, and friends are sometimes unsure of how exactly they can help.
"People who feel mentally unwell might find it hard to discuss how they're feeling but friends can provide support."
The Mental Health Foundation has produced a new booklet called Keeping Us Going, which offers help and guidance for people with mental health problems and their friends.