Fluorouracil (5FU) is a type of chemotherapy used to help treat several types of cancer. Side effects may include dizziness, nausea, and low blood count, among others.

Cancer is a disease where cells in the body become abnormal and begin to grow and divide out of control. The American Cancer Society (ACS) estimates there will be just over 1.9 million new cancer diagnoses in the United States in 2022.

There are many different treatments available for cancer. One that you may have heard of is chemotherapy, which uses drugs to kill cancer cells or slow their growth. Fluorouracil is a type of chemotherapy.

Below, we’ll explore what fluorouracil is, how it works, and its potential side effects.

Fluorouracil (5FU) is a type of chemotherapy drug. It’s typically used in combination with other cancer drugs to treat several different types of cancer, including:

Whether or not 5FU is used as a part of your cancer treatment depends on several factors like the type of cancer, its stage, and your age and overall health.

Who should not receive 5FU chemotherapy?

People with a deficiency of an enzyme called dihydropyrimidine dehydrogenase (DPD) should not receive 5FU. Because DPD helps break down 5FU in the body, a deficiency in this enzyme can lead to life threatening complications.

Before starting treatment, your healthcare team may have you complete a blood test to determine your DPD level. Depending on how much DPD you have, you may either need to completely avoid 5FU or have a lower dose than usually prescribed to prevent severe side effects.

5FU is also not recommended if you:

There are several different types of chemotherapy drugs. Each type works in a different way to treat cancer.

5FU is a type of chemotherapy drug called an antimetabolite. Because antimetabolites are similar but different to molecules that occur naturally in the body, they can interfere with the growth and division of cancer cells.

In the case of 5FU, the drug is similar to one of the building blocks of DNA. When it’s substituted into the DNA of a cell, it blocks the cell from copying its DNA. As DNA needs to be copied before a cell can divide, the cell dies before it can do so.

5FU is most effective when used in combination with other drugs

By itself, 5FU may not be very effective. This is because cancer cells have several mechanisms by which they can become resistant to 5FU.

5FU is far more effective when combined with other cancer drugs. For example, when 5FU is used alone for advanced colorectal cancer, the treatment response rate is 10% to 15%. This increases to between 40% and 50% when it’s given with other drugs.

Folinic acid, typically called leucovorin calcium, is the drug almost always given with 5FU. Leucovorin improves the binding of 5FU to the enzymes in cancer cells. This allows the chemotherapy to stay inside the body for a longer time, giving it the opportunity to kill more cancer cells.

In many cases, 5FU is delivered directly into your bloodstream (intravenously), which means it can reach many areas throughout the body. This is called systemic treatment.

There are several ways that you can receive 5FU intravenously. These include the following:

  • Peripheral IV line: This is a thin tube placed into a vein in your arm or in the back of your hand.
  • Central venous catheter (CVC): A CVC consists of a narrow, flexible tube called a catheter that’s inserted through your chest and into a large vein near your heart. A port is used to deliver medication into a CVC.
  • PICC line: A PICC line is a catheter that’s inserted into a vein in your arm and threaded to a large vein near your heart. The end of the catheter sticks out through your skin, and its opening is covered with a special cap.
  • Bolus dose (IV push): Many people receiving chemotherapy regimens containing 5FU get an IV bolus as a loading dose (which takes a couple of minutes). This is followed by a connection to a home infusion system where the drug runs slowly over many hours (approximately 22 to 46 hours).

5FU can be given as a drip, which often includes the use of a pump that delivers the drug over a set period of time. You may also receive 5FU continuously via a special portable pump that’s connected to a CVC or PICC line.

Chemotherapy is given in cycles. One cycle consists of treatment (sometimes for a few hours or up to a few days) followed by a rest period (between 2–3 weeks usually) to allow your body to recover from the treatment and its side effects. For 5FU, you’ll typically have six cycles of treatment.

5FU can also be given as a cream that you apply directly on your skin. This is called a topical treatment. Topical 5FU is used to treat some types of skin cancer.

5FU is also cell-cycle specific

5FU is cell-cycle specific. That is, it acts in a specific phase of the cell cycle: the S-phase. This means that giving 5FU at a lower dose over a longer time — 46 hours, for example — helps kill more cancer cells because it catches more of them in the S-phase over time.

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Chemotherapy targets fast-growing cells in your body. While this includes cancer cells, it can also affect healthy cells in areas like the digestive tract, hair follicles, and bone marrow.

Because of this, there are several short-term side effects that you may experience while taking 5FU. These include:

Sometimes, 5FU can negatively affect the heart, causing symptoms that may include chest pain, shortness of breath, or irregular heartbeat. Because of this, your doctor may want to monitor your heart function while you’re on 5FU.

When 5FU is applied to the skin, it can also potentially lead to skin that’s:

  • irritated
  • red
  • itchy
  • painful or sore
  • flaky or crusty
  • sensitive to light

Overall, the short-term side effects of 5FU should begin to ease after you stop taking it. Contact your doctor if you develop concerning side effects while taking 5FU.

There are also a few potential long-term side effects of 5FU. While short-term side effects typically go away after your treatment ends, these side effects can linger or persist.

5FU can weaken the immune system. Because of this, you’ll likely need to wait for a few months after your treatment concludes before getting live vaccines, such as the:

Is it OK to have vaccines when I’m undergoing 5FU chemo treatment?

After consultation with your doctor, it’s generally OK to receive some types of vaccines when you’re getting chemotherapy, such as the:

People undergoing chemo should not get live vaccines like the flu nasal spray vaccine and the chickenpox vaccine. That’s because live vaccines can potentially cause serious problems for people with a weakened immune system.

If you have a weakened immune system, your body may not create a good defensive response to vaccines. Because of this, in some situations, your doctor may recommend waiting for at least 3 months after your treatment ends to get vaccinated.

How will 5FU treatment affect fertility?

It is possible to experience fertility problems following 5FU treatment. But compared to other types of chemotherapy drugs, 5FU is associated with a lower risk of infertility in both females and males, according to the ACS.

Nevertheless, if you have concerns about fertility and your cancer treatment, talk with your doctor. Based on your individual situation, they may recommend freezing eggs or sperm prior to starting your treatment.

Is it OK to breastfeed while undergoing 5FU treatment?

Generally speaking, you should not breastfeed when undergoing systemic chemotherapy with 5FU. But if your treatment is intermittent rather than continuous, you may be able to breastfeed. Always consult with your doctor first.

If you’re using 5FU topically, you can still breastfeed as long the 5FU is not being applied to your breast or nipple. Also take care to prevent your infant’s skin from coming into contact with any 5FU cream on your body.

5FU is a type of chemotherapy drug that interferes with the division of cancer cells. It can be used for several different cancers, including, but not limited to, breast cancer, colorectal cancer, and skin cancer.

Like any chemotherapy drug, 5FU is associated with both short- and long-term side effects. Your doctor will inform you of these before you begin your treatment.

Starting chemotherapy can feel overwhelming. If you have concerns about your treatment plan, discuss them with your doctor. Remember that they’re here to answer your questions and help put you at ease.