Fluorouracil (5-FU): What to Expect, Side Effects, and More
Fluorouracil (5-FU) is a type of chemotherapy medicine that gets in the way of cell division by blocking a cell from copying its DNA (instruction manual of the cell). 5-FU is prescribed to people with early- or advanced-stage breast cancer, as well as people with other types of cancer. 5-FU is often used in combination with other chemotherapy medicines, including in the CMF chemo regimen.
See fluorouracil prescribing information.
How fluorouracil works
Fluorouracil is a type of chemotherapy called an antimetabolite. It blocks cancer cells' ability to make copies of DNA. Without a copy of DNA, a cell cannot divide. 5-FU causes the cancer cell to die as it gets ready to divide.
Gemzar, methotrexate, and Xeloda are other antimetabolites.
What to expect during Fluorouracil treatment
Fluorouracil is given as an intravenous injection.
The recommended dose of fluorouracil is 500 mg/m2 or 600 mg/m2 on days one and eight every 28 days for six cycles.
When used with other chemotherapeutic agents to treat breast cancer, it can be given on different schedules — in some cases every 21 days, or on the first and eighth day of a 28-day treatment cycle. When 5FU is given by itself, it may be administered on a weekly schedule, and the dose may be higher.
Fluorouracil side effects
Like almost all breast cancer medicines, fluorouracil can cause side effects, some of them severe.
The most common side effects of fluorouracil are:
taste changes, metallic taste in mouth during infusion
irregular periods — this can include temporary cessation (usually resume after medication is completed) or permanent cessation of menstrual periods depending on your age and other factors
Fluorouracil can cause fetal harm, so taking it while pregnant is not recommended. Effective contraception should be used during treatment.
People with a mutation in the DPYD gene have a much higher risk of severe, sometimes life-threatening side effects from fluorouracil. Researchers estimate that out of 1,000 people who receive fluorouracil, at least 20 might have a DPYD mutation.
The DPYD gene provides instructions for making an enzyme called dihydropyrimidine dehydrogenase (DPD), which is involved in the breakdown of molecules called uracil and thymine when they are not needed. People with a DPYD mutation may have complete or partial DPD deficiency.
Some people with a DPYD mutation may have seizures, intellectual disabilities, and autistic behaviors that affect their ability to communicate. Other people have no signs or symptoms.
It’s recommended that people with a DPYD mutation either receive a lower dose of fluorouracil or receive a different medicine. In 2020, the European Society for Medical Oncology (ESMO) recommended genetic testing for anyone prescribed fluorouracil. The National Comprehensive Cancer Network (NCCN) and the American Society of Clinical Oncology (ASCO) do not recommend testing for DPYD mutations in the United States.
In 2025, the U.S. Food and Drug Administration updated the product label to include a warning about the risks of administering fluorouracil to people with DPD deficiency.
If fluorouracil is part of your treatment plan, it’s a good idea to talk to your doctor about your family’s medical history and ask if DPYD testing makes sense for you.
Real patient reviews
Members of our community discuss their experiences with fluorouracil in the community forum. Here are some of their comments. Remember, it's very important to talk to your doctor about any side effects you're having and ask about ways to manage them.
“They started me on cyclophosphamide + methotrexate + fluorouracil (CMF) a three-drug combo. I started last Friday. It is an infusion that takes about 2 hours and I will receive it every three weeks. So far, no side effects.” —Texasrose53
“I have wonderful things to say about the cytoxan, methotrexate, 5-FU combo. I had six months of it in 1990. [Afterward,] I had over 22 years with no evidence of cancer. I did lose my hair at about the five-month mark. Side effects for me were pretty mild. I recall only two days of throwing up. Hair, eyebrows, and lashes all came back.” —SchnauzerMom
“My hands and feet were painful for the four days of each 5-FU round — hand-foot syndrome. That went away, but gradually my toes and soles of my feet became numb 6-12 months later.” —16Christine
Read more in our fluorouracil discussion forum.
Paying for Fluorouracil
The cost of fluorouracil may vary. The price you’ll pay depends on the drug manufacturer, your health insurance provider and plan, where you live, and the pharmacy you use.
Note: Your insurance company may require you to get prior authorization before covering fluorouracil.
Learn more about financial assistance and medicine cost-lowering tips.
— Last updated on March 29, 2025 at 6:19 PM