Heart disease is the biggest killer in Australia, yet it seems many of us are unaware we are at risk.

New research published in the Medical Journal of Australia has found that 20 per cent of Australians between the ages of 45 and 74 are at high risk of having a heart attack or stroke over the next five years.

The study's authors said those at high risk of cardiovascular disease (CVD) could benefit greatly from being prescribed a mixture of blood pressure and cholesterol-lowering medications.

But the researchers also found that many of these people were unaware of their risk and were not taking medications that could save their lives.

Responding to the study, Professor Anthony Rodgers from the George Institute for Global Health said the findings were a "real wake-up call", highlighting just how much potential benefit was being left on the table.

"These drugs ... we know a lot about them. In the large majority of cases they're very safe and cause little to no side effects, but they're not being used as widely as they should be," he said.

"[Cardiovascular disease] is our biggest killer, but there a lot of moderate to high risk people walking around unprotected in Australia."

So how do doctors calculate a person's risk of heart disease?

Your absolute risk score

"There's a number of factors that you've got to take into account ... all at the same time," Professor Rodgers said.

"The leading ones are your age, your sex, what your blood pressure is, what your level of cholesterol is, whether you smoke and whether you've got diabetes."

As well as asking you questions about your lifestyle, your GP will check your blood pressure and order a blood test to check your cholesterol and whether you have diabetes.

The results will typically be put into an online calculator — which calculates your risk of having a stroke or a heart attack over the next five years, as a percentage

The figure generated is what doctors refer to as your "absolute risk score".

"Once you've got your risk estimated, the doctor will tend to think about it in categories and in general, they will regard over 15 per cent over five years as a high risk," Professor Rodgers said.

A figure between 10 and 15 per cent puts you in the moderate risk category and less than 10 per cent in the low risk category.

So who should be asking their GP to calculate their cardiovascular disease risk?

Heart Foundation guidelines state that if you are over 45 years of age you should have your absolute risk (or heart health) checked. If you are an Aboriginal or Torres Strait Islander person, the age is lower, and you should get a check once you pass the age of 35.

The Heart Foundation's Professor Gary Jennings said there were also other groups of people who should consider whether tests were necessary prior to these ages, in consultation with their doctor.

"Those who've got a really strong family history of heart disease or some sort of vascular disease, and of course those who have ever been diagnosed with a heart or vascular problem, need close follow up of those risk factors," he said.

Who needs medication?

Professor Rodgers said when deciding whether to prescribe medication, doctors tended to think about the issue in terms of risk categories.

"In general, they will regard over 15 per cent (risk of heart attack or stroke) over five years as a high risk," he said.

"So in that group ... there is a particular imperative to think about preventative treatments, as that's where you've got the most to gain."

Professor Rodgers said for people with an absolute risk factor of under 15 per cent, the question of whether to prescribe preventative medicine was more likely to be weighed up against potential side effects.

The kind of drugs commonly used to lower your risk of CVD fall broadly into two categories — those that work to lower cholesterol and those that reduce blood pressure.

Statins are the most common type of drug used to lower cholesterol.

But there are a range of drugs used to lower blood pressure.

"There's really five main classes that are used widely. There's diuretic-type drugs, ACE inhibitors, angiotensin receptor blockers, beta blockers and calcium channel blockers," Professor Rodgers said.

He said there was solid evidence to indicate that statins and blood pressure medications delivered the biggest benefits across a large number of patient groups.

"Most people [at risk of CVD] have blood pressure or cholesterol that's too high," he said.

"Unless your blood pressure levels are low or your cholesterol levels are low, it's likely that you're going to benefit from getting those levels down with these types of drugs."

As well as taking these medications, those with any kind of raised risk will be encouraged to lifestyle changes such as improving diet and exercise.

What about side effects?

As with many drugs, statins and blood pressure-lowering drugs do have potential side effects.

But Professor Rodgers said there were very few side effects comparable to the severity of a heart attack or a stroke.

"A life-threatening side effect from blood pressure-lowering drugs or statins is exceptionally unusual — one in tens of thousands or hundreds of thousands of people," he said.

"That's an important thing to remember. You're balancing off mild to moderate symptomatic problems with something that could be as life changing as a stroke or a heart attack."

The most common side effects for those taking statins were muscle aches and weakness, and these were only reported in very small numbers.

Some studies have linked statins to type 2 diabetes, but the risk posed by cardiovascular disease is believed to be far more significant.

The most common side effects for blood pressure lowering drugs are dizziness or faintness, but again this only happens with a small number of people.

How much does lifestyle impact on heart health?

Raised cholesterol levels, high blood pressure and smoking are what experts refer to as the "big three" factors behind CVD.

"Three quarters of heart disease or stroke is due to high levels of blood pressure or cholesterol or to smoking," Professor Rodgers said.

So sustained changes to diet, physical activity levels and smoking status can all have a big impact play a big role in terms of your risk of having a stroke or a heart attack, and also in your treatment outcomes.

"It's really essential to preventing heart disease and stroke," Professor Rodgers said.

"Improving physical activity levels, reducing salt in your diet, reducing saturated fat in your diet etc ... those are the ways to prevent heart disease in the long term."

The challenge however, can be making the large, sustained changes required in order to have a big impact on your health.

"That's quite challenging in environments like Australia, where for example there's so much salt in the food all around us ... it's difficult to cut out salt substantially. But it's really the corner stone of cardio vascular prevention," Professor Rodgers said.

Another area in which people often fall down is in thinking that because some aspects of their lifestyle are considered "healthy", their CVD risk factor must be low.

But Professor Rodgers said it was possible to be very fit while still having very high cholesterol, which he referred to as a "silent risk factor".

"An athlete can have been eating high saturated fat for 20 to 30 years and appear super fit, but that still doesn't mean that they're not clogging up their arteries," he said.

"It's entirely impossible to see that externally, until they have their heart attack or stroke. It's really only obesity that's visible externally in terms of these risk factors."