There has been an "alarming" rise in the rates of self-harm in England, research suggests.
A Lancet Psychiatry study found that in 2014, 6% of people had self-harmed, up from 2% in 2000 - with the highest rate in those aged 16-24.
But the number getting no help remained at about 50% - and experts said more support was needed.
And there are concerns self-harm could become more serious as people age, leading to suicidal behaviour.
The term self-harm describes when people hurt themselves, perhaps by punching, hitting or burning, as a way of dealing with difficult feelings or overwhelming situations and experiences.
The triggers can be complex, but experts suggest academic pressures, and problems such as bullying or body image, are increasingly significant factors.
Emma Thomas, chief executive of the charity YoungMinds, called the Lancet Psychiatry figures "alarming", adding: "It's far too difficult for children and young people to get mental health support before they reach crisis point."
'You're never alone'
Dr Sally McManus, of the National Centre for Social Research who led the study, said she was struck by the numbers who did not have contact with medical or psychological services.
"At least half of people who self-harm don't get that sort of support. It's important we offer alternative, safer ways of coping."
She said that could include giving people problem-solving skills, and promoting the benefits of exercise or cognitive behavioural therapy techniques.
"We need to place the emphasis on prevention, and prevent self-harm becoming embedded as a way of coping with emotional stress."
The study looked at data from a series of mental health surveys of people across England, with 7,243 responses in 2000, 6,444 in 2007 and 6,477 in 2014.
Among 16 to 24-year-old women in 2014, 19.7% reported having self-harmed at some point in their life, compared with 11.7% in 2007 and 6.5% in 2000.
This increase was mainly because of rises in self-cutting and use of self-harm as a way of coping with unpleasant feelings.
Experts say that intervention, as well as helping people with their situation at the time, may prevent future suicidal behaviour.
Prof Louis Appleby, who co-authored the paper and leads the National Suicide Prevention Strategy for England, said there was a risk that self-harm could become a normal way of people coping with difficulties in their lives as they got older.
"The self-harm they have learned may become more serious and more likely to have a fatal outcome.
"Young people need health and educational services to be available, and health professionals need to discuss self-harm with young people and encourage them to find safer ways of coping."
Sophie Beer, manager of Addaction's Mind and Body self-harm service, said: "Our message to young people is, 'You're never alone. Don't be afraid to reach out for help if you are struggling.'"
A Department of Health spokesperson said mental health was a key priority for the government.
It said its long-term plan for the NHS would give 380,000 more adults access to psychological therapies by 2023/4, and 345,000 more children and young people greater support in the next five years.
It also said it was piloting a four-week waiting time standard for treatment, training a new dedicated mental health workforce for schools, and "teaching pupils what good mental and physical health looks like".
What adults can do to help a child who is self-harming:
- Show you understand
- Talk it over
- Discover the triggers
- Build their confidence
- Show you trust them
- Choose who you tell carefully
- Help them find new ways to cope
How to spot warning signs:
Look for physical signs such as cuts, bruises, burns and bald patches from pulling out hair.
The emotional signs are harder to spot:
- tearfulness and low motivation
- becoming withdrawn and isolated, for example wanting to be alone in their bedroom for long periods
- sudden weight loss or gain
- low self-esteem and self-blame
- drinking or taking drugs
For further information about self-harm, click here, or visit: