Seeing your GP
You should arrange to see your doctor as soon as possible if you have any of the following changes.
It must be stressed that these signs and symptoms listed can also be linked with other medical conditions or illnesses.
It doesn't necessarily mean that you have cancer.
Get to know your body and check yourself regularly. Become aware of any lumps or changes under the skin, e.g. Breast, Testicles and under the arms.
If you have any moles check them regularly for any changes i.e. bleeding, itching, altering shape or size, these can include sores that do not heal, darkening/reddening of the skin and/or excessive hair growth. Ensure you keep covered up in the sun and use appropriate sun protection.
If you find that you have lost a lot of weight in a short time (and are not on a diet) and are also feeling more tired, then you need to go and see your GP to get it checked out.
If this continues for more than three weeks it may be associated with lung or one of the cancers of the head and neck, although another cause could be linked with chest infections or active/passive smoking.
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Change in bowel habits 
Recent and unexplained e.g. Diarrhoea, constipation or blood in the bowel movements. Become more aware with what is normal for you and eat a well balanced diet.
Pain with passing water, blood in the urine or change in bladder habit i.e. increase or decrease in visits to the toilet, difficulties in passing urine need checking out with your doctor. Remember before you panic it could be a Urinary Tract Infection.
Bleeding in between menstrual periods, after intercourse or after the menopause, may indicate a problem with the lining of the womb (uterus) or the neck of the womb (cervix). Be aware of what is normal for you and go for regular smear tests, as this can help in detecting early stages of cervical cancer.
Indigestion that will not go away or just keeps coming back and any difficulties with swallowing. These symptoms may also be caused by other medical conditions, e.g. an ulcer which you may already be receiving treatment for. Let your GP know of any change.
Ulcers or red/white patches in the mouth that do not heal. Regular mouth care e.g. dental check-ups and looking after your teeth and gums can lead to early detection of any problems related to your mouth.