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How can cancer kill you?

Firstly, it is important to say that not all cancers kill you. Overall, more than 50% of people diagnosed with cancer live for more than 5 years. Some cancers have survival rates of more than 90%. Secondly, cancer at an early stage does not kill you. So, a lot of effort is put into early diagnosis when treatment is likely to work best. Cancers can cause death in more than one way. So there is no single answer to this question. It really depends on the type of cancer you have and which parts of your body are affected. There are some examples below.

Some types of cancer can spread to take over part of the body that does something essential for life. For example, if a cancer is growing in part of the digestive system, it can block it so that food cannot go through the intestines. If food cannot pass through, then your food can't be absorbed. 

If cancer affects the lungs, then eventually there is not enough healthy lung tissue to allow you to absorb enough oxygen. Or the cancer can block off part of the lung. This part then collapses and often becomes infected. If you have advanced cancer, you may not have the strength to fight off an infection, even with strong antibiotics. So the infection can eventually lead to death. 

The human body has very finely balanced limits of certain body salts and chemicals. A cancer that has spread to the liver or bones can upset this chemical balance. The liver is the chemical factory of the body. It carries out many different tasks and is very important in maintaining the balance of body chemicals. 

Cancer in the bones can affect the calcium balance of the body. If calcium levels go up or down in the blood, it upsets the whole chemical balance. Cancer in the bones can cause a lot of calcium to be released into the bloodstream. Normally the body has systems to correct this sort of imbalance. But when the imbalance becomes too great the systems don't work any more. There is  treatment to bring calcium levels back to normal, but these only work for a limited time. Then unfortunately the calcium levels will rise in the blood. If calcium continues to go up, it will cause you to become unconscious and eventually die. 

If cancer cells take over your bone marrow, eventually you won't have enough healthy bone marrow to make blood cells. If you haven't got enough red blood cells, you won't have enough oxygen circulating around your body. A drop in white blood cells means you have less resistance to infection. A drop in platelets means you are at greater risk of abnormal bleeding. If a blood vessel in a vital part of the body is damaged it can be life threatening. For example bleeding in the brain is a stroke, which can be fatal if the body can't control it. 

Some cancers make particular substances which upset the body balance. This can cause problems such as severe weight loss or dehydration, which will eventually overwhelm the natural balancing systems of the body. 

Many treatments can control cancer for a long time, even if they can't cure it. But if a cancer continues to grow, then unfortunately it can become too much for the body to cope with. 

Although this is a difficult subject for people to talk about (including for some doctors and nurses), it may help you to ask your specialist doctor or nurse about how you or your relative may die. It is something most people worry about at some point. Talking about the way the cancer is affecting your body can help to lessen at least some of those worries.

Many people are relieved to find out that they (or their relative) are likely to become unconscious shortly before they die. What you imagine may happen before death is often far worse than the reality. It is important to remember also that very good pain control is available and no one with cancer should die in pain.

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Updated: 14 March 2014