Up to one in five patients is regularly missing GP appointments in Scotland, new research reveals.
A study of more than 500,000 people in the country found poorer patients living in affluent areas were the most likely to miss an appointment.
"No-showers" tended to be aged 16 to 30, or older than 90, according to the researchers from Lancaster, Glasgow and Aberdeen Universities.
The study findings are published The Lancet Public Health journal.
There is no centrally collected data on total number of GP appointments or how many of them are missed.
The study found 19% of patients missed more than two appointments in a three-year period.
Those appointments that were booked two or three days in advance were more likely to be missed than those made two weeks in advance.
Social deprivation was the biggest predictor.
David Ellis, of the University of Lancaster, one of the lead authors of the study, says the research has identified where doctors' surgeries need to focus their efforts.
"Some of the solutions might include practices learning to better manage patients who are more likely to not attend.
"So for example that might mean giving more appointments on the day than say, two to three days in advance.
"And because we've already got a kind of profile of what those patients might look like who are more likely to not attend, that's where the more targeted interventions could be pushed."
But Stockport GP Ranjit Gill believes there has been a shift in how the health service is seen by a younger "I want it now" generation.
"The NHS is now, for our younger population, seen as a consumer service, a bit like John Lewis and so perhaps valued differently to the way our older population see the NHS.
"I can't think of the last time one of my older patients ever missed an appointment."
And Dr Gill points out that as well as the financial loss to the health service, a missed GP appointment also represents a loss of valuable time.
"We have to check for each missed appointment that there wasn't a worrying reason behind that missed appointment whether it be mental health, safeguarding issues or other welfare concerns about patients.
"That takes time and that's a lost opportunity again, for that patient and other patients as well."
Prof Helen Stokes-Lampard, chairwoman of the Royal College of GPs, says it can be frustrating for doctors when patients don't turn up to their appointments.
"Whilst practices will always try to offer appointments that are timely and convenient for patients, the current resource and workforce pressures we are facing, with GPs conducting more consultations than ever before to meet increasing demand, is making this more and more difficult.
"GP practices across the country are already implementing some successful schemes to reduce missed appointments, from text messaging reminders to better patient education and awareness posters detailing the unintended consequences of a patient not attending.
"But ultimately, we need NHS England's GP Forward View - promising £2.4bn extra a year for general practice and 5,000 more GPs - to be delivered in full and as a matter of urgency.
"And we need equivalent promises made and delivered in Scotland, Wales and Northern Ireland, so that we can deliver the care our patients need, whatever their circumstances, and wherever in the country they live."
In 2014, NHS England estimated that more than 12 million GP visits are missed each year in the UK.
That could cost the health service in excess of £162m per year.