A quarter of the population take up half of all GP appointments and hospitalisations, a new study has found.
New research found that more than a quarter of adults in England have more than one long-term illness such as high blood pressure or diabetes.
And the majority of GP consultations, prescriptions, and hospital admissions were associated with patients with so-called multimorbidity, according to experts.
Researchers said that NHS services designed to treat single illnesses need to be redesigned to cope with the rising tide of people with multiple long-term conditions.
The authors of the study, published in the British Journal of General Practice, point out that patients with numerous long-term conditions "often have complicated medical needs, including understanding and managing multiple illnesses and complex medication regimens".
After analysing data on more than 400,000 GP patients in England, the researchers found that 27.2% have more than one long-term condition.
Just over half (52.9%) of GP consultations are devoted to patients with more than one long-term health problem.
And 78.7% of prescriptions were given to patients with multimorbidity.
Among the patients studied, 155,488 patients (38.5%) were admitted to hospital at least once during the four year follow-up period. And those with multimorbidity accounted for 56.1% of these admissions.
The authors, from the universities of Cambridge, Bristol and Utah in the US, wrote: "Essentially, one quarter of the population is accounting for more than half of health service utilisation.
"Multimorbidity was highly associated with increased rates of GP consultations, prescriptions, and hospitalisations, which highlights the disproportionately large demand that patients with multimorbidity place on the UK's overburdened healthcare system."
They added: "As the majority of healthcare encounters are now with patients with multimorbidity, all health professionals must be trained to manage the cumulative effects of more than one chronic condition.
"Many 'single-disease' services in primary and secondary care will need to be redesigned to treat patients with multiple morbidities."
Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said: "This large-scale, comprehensive research is further evidence of the increasing complexity of cases that GPs are dealing with, and the inadequacy of the standard 10-minute consultation.
"GPs need much more time with our patients with complex needs, so that we can properly consider their unique circumstances - including the different conditions they are living with - and develop the most appropriate treatment plan for them.
"This simply isn't possible in 10 minutes, but offering longer appointments means offering fewer appointments and our patients are already waiting too long to secure time with their GP.
"We need to see NHS England's GP Forward View, which promises £2.4bn extra a year for general practice, 5,000 more GPs, and for every practice to have access to one of 3,000 new mental health therapists, delivered in full and as a matter of urgency."
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