Risks and causes of cancer of unknown primary (CUP)
Cancer of unknown primary (CUP) is not a single disease. There are more than 200 different types of primary cancer. So doctors can't be specific about the possible causes or risk factors of CUP. This is because they don't know where the cancer started. This means that they don’t know which type of cancer it is.
General risks and causes of cancer
Different types of cancer have different risks and causes. For example, the biggest cause of lung cancer in the UK is smoking. And we know that overexposure to ultraviolet light from the sun or sun beds increases your risk of skin cancer.
Cancer is a disease usually caused by many different factors coming together. Our risk of cancer depends on a combination of our genes, environment and lifestyle.
Age
CUP is more common in older people. In the UK, almost 60 out of 100 people (almost 60%) diagnosed with CUP are 75 and over.
Smoking
Smoking is the biggest preventable cause of cancer in the UK. This includes breathing in other people’s cigarette smoke.
There is some evidence that smoking also increases the risk of developing CUP.
Even light or occasional smoking increases the risk of cancer. But your risk increases more the longer you smoke and the more you smoke. Stopping smoking is the best thing you can do for your health.
Drinking alcohol
There is some evidence that suggests drinking alcohol may increase the risk of CUP. Cutting down on alcohol has lots of benefits, including reducing your cancer risk.
The NHS recommends both men and women drink no more than 14 units of alcohol a week.
Obesity and weight
Overweight and obesity is the second biggest preventable cause of cancer after smoking. Losing weight or avoiding putting on more weight can help reduce the risk of cancer.
There is some evidence that people who are overweight have an increased risk of developing CUP.
Other possible causes
Stories about potential causes are often in the media. It isn’t always clear which of the ideas reported are supported by good evidence.
You might have heard about a possible cause which we haven’t included here. This is either because there is no evidence to support it, or because it is not fully clear what the evidence shows.
How you may feel
People diagnosed with cancer often feel strongly that they must find out what caused it. But for many types of cancer there is no obvious cause.
Cancer can just happen because of bad luck. Over years, a cell gathers changes to its genetic code 
that eventually make it become cancerous.
Cancer causing chemicals, such as those in cigarettes, may trigger one or two of the changes. But often, something just goes wrong in the DNA of the cell by itself – a bit like a spelling mistake in the genetic code. The damaged cell then begins to grow and divide much more quickly than normal cells. Or it doesn't die off as a normal cell would. The overgrowth of cells forms a tumour.
Being diagnosed with any type of cancer is difficult to accept. You may find yourself spending a lot of energy trying to work out what caused it.
Remember that often the cause of the cancer is never found. And if doctors do find the cause, this is unlikely to change your treatment or how well it works.
Coping
Everyone copes with this in their own way. You may find it helpful to focus your energy on getting through your treatment and staying as well as you can.
For support and information, talk to Cancer Research UK's information nurses. You can call them on freephone 0808 800 4040, from 9am to 5pm, Monday to Friday.
