As we get older, the likelihood grows that we will develop more than one chronic disease. With age comes a suite of non-infectious - long-term conditions like diabetes, hypertension, cancer, arthritis, stroke, obesity, and depression.

Yet, current clinical practices have struggled to keep pace with the rise of what is known as multimorbidity, or having two or more chronic conditions at the same time.

And it is having serious financial implications.

New research shows that the standard practice of tackling each disease in isolation is putting a huge strain on both household finances and countries' health systems.

A global study led by the University of Melbourne and the National University of Singapore has found that this approach leads to the prescription of unnecessary medicines, resulting in patient expenses that are disproportionate to the number of conditions they have.

"Many people have a chronic disease, and most have more than one," says senior author Dr John Tayu Lee from the Nossal Institute for Global Health, University of Melbourne. "The challenge we face is that around the world patients are having to navigate fragmented health systems.

"If I have a stroke or depression, I'll see specialists in different areas, and they will prescribe different drugs. The costs start to add up. In some countries the cost of drugs is the weakest link. This is the area that patients still need to pay out of their own pockets."

Published in the international journal BMJ Global Health, the research was conducted by the National University of Singapore, the University of Melbourne, Imperial College London, Harvard University, University of York, and Luxembourg Institute of Socio-economic Research.

The team analysed the raw data from 14 global studies that measured out-of-pocket expenditures – or costs not covered by insurance – of patients with multimobidity.

As the number of chronic diseases increased from none to one, researchers found that the annual out-of-pocket expenditure grew by an average of 2.7 times. An increase from one to two conditions and from two to three increased average costs by 5.2 and 10.1 times, respectively.

The increasing costs mean that patients can afford fewer medical services and many stop taking their medicines as a result. The elderly and low-income groups are particularly at risk.

"It is a burden for the patients," says Dr Lee. "And if you don't take your medicine, it creates a vicious cycle. It means they don't manage their condition effectively, meaning they will probably pay more in medical costs in the long term.

"The patient might not be able to go back to work, meaning lost productivity. So, it has wider implications for society and the economy too."

It is an issue that strikes patients in all countries, regardless of the health system. The team looked at data from the United States, Canada, Australia, South Korea and India.

In Australia, for example, the Pharmaceutical Benefits Scheme (PBS) under Medicare covers part of the cost of prescription medicines for patients' chronic diseases.

However, this coverage varies with the type of medicine and is capped.

The experts say that the PBS can be insufficient to cover costs, leaving patients with multiple chronic diseases and complex treatment regiments, out-of-pocket.

In the United States, there are other barriers. The Medicare prescription drug benefit, commonly known as Medicare Part D, has made improvements to reduce patient expenditure on prescription medicines in the recent years.

However, according to the researchers, some patients with multiple chronic conditions may still have high co-payments for their medicines, and details of plans may change often.

The Medicare's online Find-A-Plan program, which is designed to help patients update their plans to minimise spending on medicines, is complicated - especially for the elderly who may be less technologically adept.

"The nature of chronic diseases is that they require long-term management and usually cannot be cured, which exacerbates the high cost to patients for medicines over a long period of time," says Dr Grace Sum Chi-En, lead author from the National University of Singapore.

"It is imperative that healthcare policymakers start recognising this problem, to design better policies that offer financial protection on medicines for chronic diseases."

While the elderly may account for a significant proportion of those with multimorbidity, the condition is not confined to a particular age bracket.

"A lot of young adults have it too," says Dr Lee. "In some countries, more than half of all adults suffer from it."

He adds that mobile populations may be particularly susceptible to falling through gaps in health insurance coverage, leaving them exposed to high costs should they develop multimorbidity.

Many migrants from the Chinese countryside to urban areas, for example, have health insurance that only covers them in their rural home villages.

"It is a challenge for health systems at all ages, globally," he says.

And policy makers are only beginning to grapple with it.

Later this year in New York, the United Nations is holding its third high level meeting on the prevention and control of non-communicable diseases. This research will be presented and discussed.

"This study provides further impetus for leaders to put people first in their response to reduce the burden of having multiple chronic conditions, by prioritising equitable access to treatment and care for all," says Katie Dain, CEO of NCD Alliance.

"Treating chronic conditions is a human rights issue, and such vast disparities in out-of-pocket expenditures are a major, unacceptable barrier to health equity."

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