Painkillers are safe when taken according to instructions, but taking too many can be harmful - and is easily done. And taking the wrong type of painkiller may even exacerbate conditions such as kidney disease or high blood pressure.

Excessive paracetamol is the leading cause of acute liver failure in the US, with about half of all cases being accidental overdoses. A 2005 University of Texas study found that 27 per cent of people with paracetamol poisoning died and 8 per cent needed a liver transplant.

Many of those who accidentally poisoned themselves did so by taking about 20 pills a day instead of the recommended eight.

Painkillers fall into three categories: opioids, non-opioids and combined analgesics.

Opioids, also known as narcotic analgesics or opiates, are used for moderate to severe pain and are usually prescribed for patients with serious injuries or terminal illnesses. They include codeine and morphine.

Non-opioids include drugs such as paracetamol and are used to help with moderate pain such as headaches and joint pains. Many can be bought over the counter. Within the non-opioid group are drugs with anti-inflammatory properties, better known as non-steroidal anti-inflammatory drugs (NSAIDS), including aspirin and ibuprofen.

Combined analgesics use mild non-opioid drugs such as paracetamol together with a small amount of opioid - such as aspirin with codeine or paracetamol with codeine. As well as treating pain, some of these over-the-counter remedies have other properties. Aspirin, for example, is used to reduce the risk of blood clots that can lead to strokes and heart attacks.

With their multiple uses, it's no surprise that painkillers and their uses leave many consumers bewildered. 'Analgesics are a very confusing issue for the public,' says Benjamin Kwong Yiu-sum, president of the Pharmaceutical Society of Hong Kong. 'There are so many products on the market that people tend to misuse them. They either don't know what kind to use or are addicted to them.'

This is a danger with prescription opiates, but even over-the-counter paracetamol can be addictive for some people on a psychological level. 'They have to take a tablet each day or they don't feel good,' Kwong says.

Cindy Lam Lo-kuen, clinical associate professor at the University of Hong Kong's family medicine unit, says dosage and frequency errors are the most common mistakes people make with over-the-counter painkillers. She says that all painkillers have adverse effects if they're not taken properly, but that some are more serious than others.

'Some painkillers, for example aspirin or other NSAIDS, are more likely to cause side effects even at the recommended dose,' Lam says. 'Paracetamol seldom causes side effects if the recommended dose is not exceeded.

'People with high blood pressure, kidney problems, heart problems, a history of gut bleeding and stomach ulcers should avoid NSAIDS, and people with severe liver diseases should avoid high doses of paracetamol.'

Family doctor Francois Fong says that a good rule of thumb for over-the-counter painkillers is not to exceed eight pills a day. 'You'd really need to take a whole packet of 20 to have bad effects,' Fong says.

With paracetamol, there's no obvious sign of overdose until liver damage has occurred and the sufferer turns yellow from jaundice.

Fong also warns against a build-up effect. 'The body responds to pain by producing endorphins - natural painkillers. So if people take painkillers on a regular basis, their body responds to them less and so they tend to take higher doses to counter pain,' he says. 'It's more important to find out the cause of the pain.'

Alvin Chan Yee-shing, a council member of the Hong Kong Medical Association, says it's crucial to get the correct diagnosis and have a follow-up assessment.

'Don't postpone the correct diagnosis,' he says. 'Painkillers are symptomatic - people might overlook the danger.'

Painkillers such as paracetamol and ibuprofen can be taken simultaneously. The latter, as part of the NSAID group, are best for anti- inflammatory purposes, such as neck or joint pain.

For cold and flu symptoms, Lam says paracetamol is appropriate for a high temperature or fever.

There's much debate in the medical world about the best time to take painkillers - particularly whether pills should be taken with food or not.

Chan says that NSAIDs may have an irritant effect and that people may prefer to take them after meals. But there are many products that don't have these side effects.

Kwong generally recommends taking painkillers with food. 'There's debate as to whether you need to take stomach-protective agents with analgesics, so we used to advise people to take food and, if they felt uncomfortable afterwards, to take stomach protection pills,' he says.

Doctors advise people to be wary of inadvertently mixing painkillers. Many cold and flu remedies contain paracetamol, and you may be taking these in conjunction with plain paracetamol pills. And for children, they recommend paracetamol rather than NSAIDs.

'Read the label and information insert in the drug package carefully,' Lam says. 'And don't give adult preparations to children.'