What Is 5-FU?
5-FU, or fluorouracil, is a chemotherapy medication used to slow or stop the growth of cancer cells. It belongs to a class of drugs called antimetabolites, which interfere with the DNA and RNA that cells need to divide. Oncologists rely on it for several common cancers, often in combination with other treatments.
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How 5-FU Works
Cancer cells divide rapidly, and 5-FU exploits that by masquerading as a normal building block of RNA. Once inside the cell, it blocks the enzymes that copy genetic material, starving the tumor of the machinery it needs to multiply. Because normal cells also divide, the drug affects some healthy tissues, which is why side effects occur.
Common Uses
Doctors prescribe 5-FU most often for colorectal cancer, where it has a long track record. It also appears in regimens for breast, stomach, pancreatic, head and neck, and skin cancers. The drug can be given intravenously or applied as a topical cream for certain superficial skin conditions, though the IV form is far more common in oncology.
Administration and Treatment Cycles
5-FU is typically administered as an infusion into a vein, either as a short injection or a longer continuous drip over several days. Treatment cycles are spaced to give the body time to recover, often every few weeks. The exact schedule depends on the cancer type, the patient's overall health, and whether other drugs are given at the same time.
Side Effects and Monitoring
Because 5-FU affects quickly dividing cells, common side effects include fatigue, mouth sores, diarrhea, skin changes, and low blood counts. Hand-foot syndrome, where the palms and soles become red and painful, is also possible. Blood counts, liver function, and kidney function are monitored regularly during treatment to catch problems early.
What Patients Should Know
Response to 5-FU varies, and not everyone tolerates it the same way. Genetic testing, such as for DPD deficiency, can help predict severe side effects before treatment begins. Patients should report new or worsening symptoms promptly, because early management of toxicity often leads to better outcomes.