By Vivienne Parry
Can't Take, Won't Take presenter
The NHS drugs bill is huge
Up to 50% of prescription medicines are not taken as directed and 20% are never taken at all.
Given that the NHS drugs bill runs at around £7 billion a year, that is an enormous waste of precious resource, not to mention the human cost.
In 2003, it is estimated that there were 10,000 heart attacks, 5,000 deaths and about 10,000 operations that would have been avoided if those who had been prescribed statins (cholesterol lowering drugs), had been taking them as directed.
Whether you take medicines or not has no correlation with age, level of education or gender, but actually varies from prescription to prescription.
There are two factors at work. Unintentional and intentional - the can't take and won't take.
Unintentional non-compliance is where we want to take a medicine but are thwarted by barriers beyond our control, such as immediate side effects, or even something as simple as not being able to get the top off the bottle.
But it is intentional non-compliance that is fascinating.
Lack of will
Professor Rob Horne, an expert in psychology in health care at Brighton University, said: "One of the main reasons why people don't take medicines is that they don't want to.
"They either make a decision either not to take it all or to take less."
These decisions are based on our relationship with doctors, and on beliefs about ourselves and medicines.
For instance, the degree to which we perceive a personal need for the treatment and how we balance this with concerns about side effects.
Beliefs are closely linked to how people think about their own illness. Julia is an example.
She has asthma and has been prescribed a preventive inhaler to use daily, plus a reliever to use if necessary.
"For most of the time I don't feel that I'm asthmatic.
"Like most people I don't want to take anything that I don't consider necessary."
So she doesn't bother with the preventer, and only uses the reliever when she is wheezy, which is often.
She is not alone. About 60% of people with asthma do not use their preventer inhalers as directed.
Their reasons are similar to Julia's and there are also misplaced fears about the consequences of inhaling steroids, even thought the does is tiny.
In a study of teenage diabetics, Professor Tom MacDonald of the University of Dundee, found that only a third were taking their insulin as directed.
"These kids used to turn up in hospital. Before we discovered they weren't using their insulin, we assumed that they had an unusual form of insulin resistance.
"One girl confessed to me that she'd found if she didn't take insulin, she could lose weight very quickly."
For some of these adolescents, medicines were a way of controlling and manipulating those around them, despite the consequences being potentially life-threatening.
We are most likely to take medicines properly when their benefits are immediate, and least likely to take them when the condition is silent or a long way down the road.
So why do 25% of transplant patients not take their anti-rejection drugs as directed?
Feeling well may be one reason. Some years after a transplant people believe, wrongly, that now their body is used to the new organ, drugs may not be as necessary.
Or they may be troubled by side effects that further alter sense of their own identity such as weight gain with steroids, or hirsutism in women.
The realisation that these beliefs are central to motivations concerning medicines has led to a move away from the paternalistic doctor knows best implications of 'compliance'.
That has been replaced by the concept of 'concordance', in which doctors have a much more open dialogue about medicines with their patients.
This isn't a problem that's going away - whatever you call it.
Can't Take, Won't Take can be heard on Radio 4 at 2100BST on Wednesday 13 July and for seven days afterwards at Radio 4's Listen again page.