At first, the cancer may not cause any symptoms as a small mass of cells (see also Cancer overview). As the cancer grows, its physical appearance can affect nearby tissue (see also cancer warning signs). Also, some cancers release certain substances and trigger an immune reaction that causes symptoms in other parts of the body that are not close to the cancer (paraneoplastic syndrome).

Sometimes the first symptom is an abnormal result of a laboratory test performed for another reason (for example, anemia of colon cancer in a routine blood test).

Cancer affects nearby tissue by growing, pushing, and then irritating and compressing. Shock often causes pain. Compression can interfere with normal tissue function. For example, lymph nodes from bladder cancer or abdominal cancer can compress the tube (ureter) that connects the kidney to the bladder, blocking the flow of urine. Lung cancer can block airflow through a segment of the lung, causing partial lung collapse and a risk of infection.

When the cancer grows in places where there is a lot of space, such as the wall of the colon or the lung cavity, there are no symptoms until it becomes quite large. In contrast, cancer that grows in a more confined space, such as the vocal cords, causes symptoms (such as hoarseness) at a relatively low frequency. If the cancer spreads (metastasizes) to other parts of the body, the same local effects of shock and compression will eventually occur, but the symptoms may be completely different in the new location.

Cancers that involve the lining of the lungs (pleura) or the sac-like structure that covers the heart (pericardium) drain the fluid that accumulates around these organs. A large accumulation of fluid interferes with breathing and pumping of the heart.

Most cancers are usually painless, but pain can be the first symptom of some cancers, such as esophageal cancer, which causes headaches, head and neck pain, and pain when swallowing. As the cancer grows, the first symptom is usually mild discomfort, which can turn into more severe pain as the cancer grows. Pain may be caused by the tumor compressing or eroding nerves and other structures. However, not all cancers cause severe pain. Similarly, the absence of pain is not a guarantee that the cancer will not grow or spread.

At first, the tumor may bleed a little because its blood vessels are fragile. Later, as the cancer grows and invades surrounding tissue, it can grow into nearby blood vessels and cause bleeding. Bleeding may be slight, undetectable, or only detectable on examination. It usually occurs in the early stages of colon cancer. Or, especially if the cancer is advanced, the bleeding can be more significant, even profuse, and life-threatening.

The location of the cancer determines the location of the bleeding. Tumors anywhere in the digestive tract can cause bleeding in the stool. Tumors anywhere in the urinary tract can cause urinary bleeding. Other cancers can bleed into the body. Bleeding into the lungs can cause a person to cough up blood.

Blood clots
Some cancers produce substances that cause excess clotting, often in the veins of the legs (deep vein thrombosis). Blood clots in the veins of the legs can sometimes travel to the lungs and cause death (pulmonary embolism). People with pancreatic, lung, or other solid tumors, and people with brain tumors may develop excessive blood clots.

Weight loss and fatigue
People with cancer often experience weight loss and fatigue, which worsen as the cancer progresses. Some people notice that they lose weight despite having a good appetite. Others may experience loss of appetite and even nausea or difficulty swallowing food. They may become very thin. People with cancer are often very tired. If anemia occurs, these people will feel tired or short of breath with little movement.

Swollen lymph nodes
Once cancer has spread around the body, it may first spread to nearby lymph nodes and become swollen. Swollen lymph nodes are usually painless and may feel hard or rubbery. They can move freely, or if the tumor is more advanced, they can adhere to the surrounding tissue or to each other.

Neurological and muscular symptoms
A cancer can grow and press on a nerve or spinal cord, causing any of a number of neuromuscular symptoms such as pain, weakness, or sensory changes (eg, tingling). When brain cancer develops, symptoms can be difficult to identify, but can include confusion, dizziness, headaches, nausea, vision changes, and seizures. Neurological symptoms may be part of a paraneoplastic syndrome.

Respiratory symptoms
Cancer can compress and block the airways in the lungs, causing shortness of breath, coughing, and pneumonia. Cancer can also cause large pleural effusions, bleeding in the lungs, and shortness of breath when anemia occurs.