Listen to the Cancer.Net Podcast: When to Call the Doctor , adapted from this content.
Cancer and cancer treatments may cause side effects that require immediate attention. However, it is often hard to know when to call the doctor. For example, you may struggle to differentiate between a common cold and a more serious infection. Therefore, it is important to ask your doctor to explain the potential side effects of your specific type of cancer and cancer treatment, as well as the circumstances under which you should call for help.
Some side effects that require immediate attention include infections, deep vein thrombosis (a potentially life-threatening blood clot), and tumor lysis syndrome (a condition that can cause organ failure).
An infection  occurs when bacteria, viruses, and, less commonly, fungi (such as yeast) invade the body’s tissues, and the immune system (which fights harmful organisms) cannot quickly destroy them. Cancer and its treatment may make it more likely that you will develop an infection. For example, a tumor in the lung might prevent normal secretions in the lung, making it easier for you to develop pneumonia. Other common sites of infection include the bladder, urinary tract, and gastrointestinal tract (which includes the stomach and intestines). Meanwhile, cancer treatments may also weaken the immune system, increasing the likelihood of an infection. For example, chemotherapy lowers the number of neutrophils (white blood cells that help fight infection).
The signs and symptoms of an infection vary depending on which part of the body is affected and whether it is a bacterial, viral, or fungal infection. However, common symptoms include a fever (a temperature of 100.5˚F or higher), chills or sweating, and a general sense of feeling unwell. If you feel fine one day but feel poor the next, monitor your temperature to see if you have a fever. Other possible signs of an infection include a new cough, a burning sensation when urinating, new episodes of diarrhea, or an ulcer (sore) on the skin.
If you show any signs of an infection, call your doctor immediately, particularly if you have a fever seven to 10 days after receiving chemotherapy. Your doctor will help you determine whether the infection is serious and will recommend treatment.
To help prevent infections, get enough sleep and proper nutrition, exercise, wash your hands regularly, and avoid contact with people who are ill. Not all infections can be prevented, though.
Deep vein thrombosis
Thrombosis is a blood clot inside a blood vessel. Deep vein thrombosis (DVT) occurs when a blood clot forms inside a vein deep in the body, usually in the legs. Although the blood clot may go away naturally, DVT becomes life threatening if the clot travels to the lungs and causes a pulmonary embolism (PE), which is a blockage of one or more of the lung’s major arteries. Signs and symptoms of a PE include shortness of breath, a cough, a fever, or chest pain that becomes worse if you take a deep breath.
A common sign of DVT is swelling in the arm or leg that becomes warm, red, and painful. Contact your doctor as soon as you notice any swelling. DVT can be treated with drugs called anticoagulants.
Some types of cancer, such as adenocarcinomas (cancers that form in gland cells), and cancer treatments, including surgery, chemotherapy, and hormonal therapy, increase your risk of DVT because they may activate the body’s blood-clotting system. Prolonged immobility (not being able to move) is another risk factor for DVT. To help prevent blood clots, get out of bed and move around as soon as you can after surgery or illness. In addition, when sitting for an extended time while traveling, periodically get up from your seat and flex your muscles. If you have had DVT before, ask your doctor about wearing compression stockings while traveling or taking blood-thinning medication before a trip.
Other risk factors for DVT include a family history of blood clotting disorders, a recent surgery, and other conditions, such as heart disease and lung disease. The risk also increases with age.
Ask your doctor about your risk of developing a blood clot or DVT and about what you can do to help prevent them. Learn more about bleeding  and clotting  problems and the American Society of Clinical Oncology’s guideline on preventing and treating blood clots .
Tumor lysis syndrome
Tumor lysis syndrome (TLS) occurs due to the rapid death of cancer cells in response to treatment. When tumor cells die, they release their contents—including potassium, phosphate, and parts of the tumor’s DNA—into the bloodstream, which may disrupt the body’s metabolism (the rate at which the body uses energy) and the concentration of electrolytes (salts in the body that conduct electricity and help the body function properly). These imbalances can damage organs—including the heart, liver, and kidneys—and may cause seizures, loss of muscle control, and death.
TLS usually occurs after chemotherapy for a fast-growing cancer, such as some types of leukemia (cancers of the blood) or lymphoma (cancers of the lymph system), and it is detected by blood and other laboratory tests.
Signs of TLS may include nausea and vomiting, shortness of breath, irregular heartbeat, cloudy urine, lethargy (lack of energy), or pain in the joints.
Risk factors for TLS include having a type of cancer in which the tumor cells divide rapidly, including Burkitt lymphoma and large-cell lymphoma (types of non-Hodgkin lymphoma), acute lymphocytic leukemia, acute myeloid leukemia, and chronic lymphocytic leukemia. TLS rarely develops in people with solid tumors. Other risk factors for TLS include having elevated levels of white blood cells and uric acid before treatment, kidney problems, and dehydration.
TLS usually occurs within 48 hours after starting chemotherapy. Rarely, TLS may occur before starting chemotherapy. Patients at the greatest risk of TLS receive chemotherapy in the hospital so that doctors can monitor them and deliver intravenous (IV) fluids to help prevent TLS. Medications such as allopurinol (Aloprim, Lopurin, Zyloprim) and rasburicase (Elitek) are also given to help prevent TLS by lowering the level of uric acid in the body.
Communicating with the doctor and other members of the health care team
Each type of cancer and cancer treatment causes different side effects, so ask your doctor whether you should watch for any other potential complications. It’s also important to discuss how and when to contact your doctor and other members of your health care team during your treatment. Ask your doctor the following questions:
- What are the possible side effects of my treatment, and which ones are considered an emergency?
- Under which circumstances should I call you during my treatment? What is your contact information?
- Under which circumstances should I contact other members of the health care team, and what are those individuals’ telephone numbers?
- What telephone number should I call after normal business hours?
- When can I expect a return call?
- Under which circumstances should I use email to communicate with you and your staff?
- When should I go straight to the emergency room?