5-Fluorouracil, commonly called 5-FU, is one of the oldest and most widely used chemotherapy drugs in the world. Developed in the late 1950s, it remains a cornerstone of treatment for colorectal, breast, head and neck, and several other cancers, and it also has a second life as a topical cream for precancerous skin lesions. Its staying power in oncology is remarkable given how much the field has changed, but 5-FU’s story is more nuanced than “old drug that still works.” How the body metabolizes it, who is genetically vulnerable to its toxicity, and why tumors eventually resist it are all areas of active research that shape how the drug is prescribed today.
How 5-FU Attacks Cancer Cells
5-FU is a synthetic molecule that closely resembles uracil, one of the building blocks of RNA. Because cancer cells divide rapidly and need a constant supply of new genetic material, they readily take up 5-FU and incorporate it into their machinery. Once inside, the drug disrupts the cell in more than one way. One of its active metabolites, called FdUMP, locks onto an enzyme called thymidylate synthase, which cells need to make the DNA building block thymidine. FdUMP forms a tight complex with this enzyme and a folate cofactor, effectively shutting down DNA production.1PubMed. Induction of thymidylate synthase as a 5-fluorouracil resistance mechanism
But thymidylate synthase inhibition is only part of the picture. Research has shown that 5-FU accumulates far more in RNA than in DNA, with treated cells containing roughly 3,000 to 15,000 times more of the drug in RNA. When researchers tested whether blocking RNA or DNA incorporation was more important, they found that 5-FU’s cell-killing ability depends mainly on its disruption of RNA processing and function.2PubMed Central. UNG-initiated base excision repair is the major repair route for 5-fluorouracil in DNA, but 5-fluorouracil cytotoxicity depends mainly on RNA incorporation This dual attack on both DNA synthesis and RNA function is part of what makes 5-FU effective across so many cancer types.
Where 5-FU Is Used in Cancer Treatment
Despite decades of newer drugs entering the market, 5-FU paired with leucovorin (a form of folate that enhances its activity) remains the backbone of chemotherapy for most patients with metastatic colorectal cancer.3PubMed. Metastatic colorectal cancer: Advances in the folate-fluoropyrimidine chemotherapy backbone Leucovorin works by raising intracellular levels of the folate cofactor that helps FdUMP bind more tightly to thymidylate synthase, making the enzyme inhibition stronger and longer-lasting.4European Journal of Cancer and Clinical Oncology. The enhancement of 5-fluorouracil antimetabolic activity by leucovorin, menadione and α-tocopherol
In practice, 5-FU almost never flies solo anymore. Modern colorectal cancer regimens combine it with other agents in protocols known by their acronyms. FOLFIRI adds irinotecan to the 5-FU/leucovorin base, while FOLFOX adds oxaliplatin. FOLFOXIRI throws in both. A phase III trial comparing FOLFOXIRI against FOLFIRI as first-line treatment for metastatic colorectal cancer found that the triple-drug combination improved response, though at the cost of greater toxicity.5PubMed Central. FOLFOXIRI (folinic acid, 5-fluorouracil, oxaliplatin and irinotecan) vs FOLFIRI (folinic acid, 5-fluorouracil and irinotecan) as first-line treatment in metastatic colorectal cancer (MCC) Beyond colorectal cancer, 5-FU-based regimens are standard in cancers of the stomach, esophagus, pancreas, breast, and head and neck.
How the Body Processes 5-FU
Here is the unsettling math behind 5-FU: roughly 80 to 85 percent of each dose is broken down into inactive waste products before it ever reaches a tumor. An enzyme called dihydropyrimidine dehydrogenase, or DPD, is responsible for this rapid breakdown. Only about 1 to 3 percent of the original dose actually exerts anticancer effects.6PubMed Central. 5-FU Metabolism in Cancer and Orally-Administrable 5-FU Drugs – Section: 5-FU Catabolism This means that small changes in how much DPD a person produces can lead to dramatic differences in drug exposure. Someone with slightly less DPD activity might get two or three times the effective dose compared to someone with robust enzyme function, even though both received the same milligrams per square meter of body surface area.
This massive variability is the central pharmacological challenge with 5-FU. Traditional dosing is calculated from body surface area, a formula based on height and weight. But body surface area tells you almost nothing about how quickly any individual patient will metabolize the drug. Two patients of the same height and weight can have wildly different blood levels of active 5-FU after receiving identical doses.
DPD Deficiency and the Risk of Lethal Toxicity
Because DPD handles the vast majority of 5-FU clearance, people who carry genetic variants that reduce DPD activity are at serious risk. Four specific variants in the DPYD gene have been identified as particularly dangerous. Carriers of these variants are estimated to be 1.6 to 4.4 times more likely to experience severe side effects and more than 25 percent more likely to experience lethal toxicity compared to non-carriers.7PubMed Central. Testing for Dihydropyrimidine Dehydrogenase Deficiency to Individualize 5-Fluorouracil Therapy
Among these variants, one splice-site mutation (known as IVS14+1G>A, or rs3918290) causes the largest drop in DPD function and is the strongest predictor of severe fluoropyrimidine toxicity. Research into additional genetic markers has identified multi-variant haplotypes that produce a comparable decrease in 5-FU clearance, suggesting that looking at single mutations alone underestimates the number of patients at risk.8PubMed Central. Genotype–phenotype correlations in 5-fluorouracil metabolism: a candidate DPYD haplotype to improve toxicity prediction Pre-treatment DPYD testing is now recommended or required in several countries, including across much of Europe, before starting any fluoropyrimidine regimen. In the United States, adoption has been slower but is growing.
Side Effects Beyond Bone Marrow
The side effects that most oncology patients associate with chemotherapy, such as nausea, mouth sores, and low blood counts, all apply to 5-FU. But two toxicities deserve special attention because they are somewhat unique to this drug.
Heart Toxicity
5-FU can trigger coronary artery vasospasm, a sudden tightening of the blood vessels feeding the heart, even in patients with no prior heart disease. Patients experiencing this may show ECG changes that look just like a heart attack, complete with elevated troponin levels, yet their coronary arteries are clean on imaging.9PubMed Central. 5-fluorouracil and cardiotoxicity: a review Other proposed mechanisms include direct damage to the blood vessel lining and toxic effects on heart muscle cells.10Interventional Cardiology Review. Coronary Artery Vasospasm Induced by 5-fluorouracil: Proposed Mechanisms, Existing Management Options and Future Directions When chemotherapy-related vasospasm is identified, treatment typically involves stopping the drug and administering calcium channel blockers or nitrates to relax the coronary arteries. For many patients, the episode resolves quickly once 5-FU is discontinued, but rechallenge is risky and usually avoided.
Hand-Foot Syndrome
Hand-foot syndrome is a painful reddening, swelling, and peeling of the palms and soles that can make everyday tasks miserable. It is not life-threatening, but it is a leading cause of dose reductions and treatment interruptions.11Clinical Cancer Research. Can Inhibiting Dihydropyrimidine Dehydrogenase Limit Hand-Foot Syndrome Caused by Fluoropyrimidines? The syndrome is more common with continuous intravenous infusion of 5-FU than with brief bolus doses. In phase III trials, rates of any-grade hand-foot syndrome with intravenous 5-FU ranged from about 3 to 18 percent, while capecitabine (an oral prodrug that converts to 5-FU in the body) produces rates between 22 and 77 percent.12PubMed Central. Management of cytotoxic chemotherapy-induced hand-foot syndrome – Section: Incidence of hand-foot syndrome Interestingly, adding a DPD inhibitor to the treatment regimen can significantly reduce hand-foot syndrome, likely by changing how 5-FU is broken down and distributed in the skin.
Continuous Infusion Versus Bolus Dosing
How 5-FU enters the bloodstream turns out to matter as much as how much is given. A major meta-analysis comparing continuous intravenous infusion against brief bolus injections found that infusion produced a significantly higher tumor response rate (about 22 percent versus 14 percent) and a modest but statistically significant improvement in overall survival. The toxicity profiles, however, were dramatically different: severe blood count drops occurred in about 31 percent of bolus patients but only 4 percent of infusion patients, while hand-foot syndrome ran in the opposite direction, hitting 34 percent of infusion patients compared to 13 percent on bolus.13PubMed. Efficacy of intravenous continuous infusion of fluorouracil compared with bolus administration in advanced colorectal cancer This trade-off means that the way 5-FU is delivered shapes not just its effectiveness but the daily experience of treatment. Continuous infusion typically requires a portable pump and a central venous catheter, which adds inconvenience but generally delivers better outcomes.
The Antidote for 5-FU Emergencies
For decades, a 5-FU overdose or an unexpected severe reaction in a DPD-deficient patient was essentially untreatable. That changed in 2015 when the FDA approved uridine triacetate, sold under the brand name Vistogard. This oral drug works as a competitive antidote: it floods the body with uridine, which competes with the toxic 5-FU metabolites for incorporation into RNA and DNA, essentially diluting the damage. In clinical studies, uridine triacetate improved survival in patients with fluoropyrimidine overdose or severe toxicity from 16 percent to 94 percent, and about a third of treated patients were able to resume chemotherapy within 30 days.14PubMed. Review of the fluoropyrimidine antidote uridine triacetate
Case reports in patients with confirmed DPYD deficiency have shown that uridine triacetate can delay the onset of severe toxicity by several weeks and shift the pattern of side effects to less dangerous forms.15PubMed. Benefit of uridine triacetate (Vistogard) in rescuing severe 5-fluorouracil toxicity in patients with dihydropyrimidine dehydrogenase (DPYD) deficiency The catch is that it must be given within 96 hours of 5-FU exposure to be effective, and it is expensive. Still, its existence has fundamentally changed the risk calculus of fluoropyrimidine therapy, turning what used to be a potentially fatal accident into something survivable.
Topical 5-FU for Precancerous Skin Lesions
Outside of oncology infusion suites, 5-FU has a completely different application as a topical cream for actinic keratoses, the rough, scaly patches on sun-damaged skin that can progress to squamous cell carcinoma. Applied as a cream (usually at 5 percent concentration, sometimes at 0.5 percent), it selectively kills the rapidly dividing abnormal cells while largely sparing healthy skin underneath.
A randomized clinical trial found that topical 5-FU reduced the number of actinic keratoses on the face and ears by about 73 percent at six months, compared to a 24 percent reduction in the control group. That benefit persisted over a 42-month follow-up period.16JAMA Dermatology. Long-term Efficacy of Topical Fluorouracil Cream, 5%, for Treating Actinic Keratosis: A Randomized Clinical Trial When lower-concentration (0.5 percent) and standard (5 percent) formulations were compared head to head in a split-face study, both produced identical complete clearance rates of 43 percent, though the higher concentration caused more skin irritation.17PubMed. Comparison of topical 5-fluorouracil formulations in actinic keratosis treatment Treatment typically involves applying the cream once or twice daily for two to four weeks, during which the skin becomes red, raw, and often quite uncomfortable before healing with noticeably clearer skin.
Why Tumors Stop Responding
Resistance to 5-FU is one of the biggest obstacles in cancer treatment, and it does not have a single clean explanation. Tumors can become resistant through a long list of biological changes, including ramping up production of the very enzyme (thymidylate synthase) that 5-FU is designed to block, altering drug transport so less 5-FU gets inside the cell, repairing the DNA damage faster than 5-FU can cause it, and evading the programmed cell death that damaged cells normally undergo.18PubMed Central. Recent Updates on Mechanisms of Resistance to 5-Fluorouracil and Reversal Strategies in Colon Cancer Treatment
More recently, researchers have turned attention to the tumor microenvironment, the surrounding tissue and immune cells that can either help or hinder chemotherapy, as well as small regulatory RNA molecules (miRNAs) and epigenetic changes that alter gene expression without changing the DNA sequence itself.19PubMed Central. 5-Fluorouracil resistance mechanisms in colorectal cancer: From classical pathways to promising processes Cancer stem cells, a subpopulation of tumor cells with heightened survival abilities, also play a role in resistance and relapse after initial treatment.20PubMed. 5-Fluorouracil (5-FU) resistance and the new strategy to enhance the sensitivity against cancer: Implication of DNA repair inhibition Overcoming resistance remains one of the most active areas of fluoropyrimidine research, with strategies ranging from combining 5-FU with DNA repair inhibitors to targeting the signaling pathways that cancer cells hijack to survive.
Capecitabine and the Shift to Oral Dosing
One of the most significant developments in 5-FU’s history was the creation of capecitabine, an oral pill that the body converts to 5-FU through a three-step enzymatic cascade. The clever part of capecitabine’s design is that the final enzyme in this cascade, thymidine phosphorylase, is concentrated in tumor tissue and in the liver, so the drug preferentially generates 5-FU where it is most needed.21PubMed. Design of a novel oral fluoropyrimidine carbamate, capecitabine, which generates 5-fluorouracil selectively in tumours by enzymes concentrated in human liver and cancer tissue For patients, capecitabine means swallowing tablets at home rather than spending hours in an infusion chair. It has largely replaced intravenous 5-FU in settings where the two drugs are considered interchangeable, particularly in adjuvant treatment after colon cancer surgery and in some breast cancer regimens. The trade-off, as noted earlier, is a higher incidence of hand-foot syndrome.
Therapeutic Drug Monitoring
Given the enormous variability in how patients metabolize 5-FU, there has been growing interest in measuring actual drug levels in the blood and adjusting doses accordingly, a practice called therapeutic drug monitoring, or TDM. Two prospective randomized trials have shown that TDM-guided dosing reduces severe side effects while improving response rates compared to standard body-surface-area dosing. One trial in patients with advanced head and neck cancer found significant reductions in severe low blood counts and mouth sores. Another, in patients with advanced colorectal cancer, showed both less toxicity and better tumor response.22PubMed Central. Therapeutic Drug Monitoring in Oncology: IATDMCT Recommendations for 5-Fluorouracil Therapy – Section: Evaluation of TDM
A larger multicenter study conducted in routine clinical practice confirmed these findings, showing that personalizing 5-FU doses based on blood levels significantly improved drug exposure consistency and appeared to lower the incidence of toxicity.23PubMed. Prospective, Multicenter Study of 5-Fluorouracil Therapeutic Drug Monitoring in Metastatic Colorectal Cancer Treated in Routine Clinical Practice Despite this evidence, TDM for 5-FU has not yet become universal practice. The testing requires a blood draw at a specific time during infusion and access to a laboratory with fast turnaround, logistical hurdles that have slowed adoption. But the direction of the field is clear: treating all patients with the same calculated dose is increasingly seen as outdated.
Use in Glaucoma Surgery
In an entirely different corner of medicine, 5-FU is used to improve the success of glaucoma filtration surgery. This procedure creates a small drainage channel to lower eye pressure, but the body’s wound-healing response often scars the channel shut. Because 5-FU inhibits the proliferation of fibroblasts, the cells that produce scar tissue, it can be applied during or after surgery to keep the drainage pathway open.24PubMed Central. 5-Fluorouracil for glaucoma surgery
In practice, surgeons either apply a sponge soaked in 5-FU directly to the surgical site during the procedure or inject it under the conjunctiva (the clear tissue covering the white of the eye) in the postoperative period when scarring is observed.25PubMed Central. Pharmacological Approaches to Modulate the Scarring Process after Glaucoma Surgery – Section: Fluorouracil (5-FU) Research combining 5-FU with bevacizumab, a drug that blocks blood vessel growth, has shown stronger anti-scarring effects than either agent alone, suggesting that future glaucoma protocols may routinely use both.26PubMed Central. In Vitro Study of Combined Application of Bevacizumab and 5-Fluorouracil or Bevacizumab and Mitomycin C to Inhibit Scar Formation in Glaucoma Filtration Surgery
A Dangerous Drug Interaction Worth Knowing About
One of the most dramatic drug interactions in pharmacology involves 5-FU and sorivudine, an antiviral drug used to treat shingles. Sorivudine is broken down into a metabolite called BVU, which powerfully inhibits DPD, the same enzyme responsible for clearing 5-FU from the body. When patients taking fluoropyrimidine chemotherapy also received sorivudine, the resulting DPD shutdown caused 5-FU levels to skyrocket, leading to fatal toxicity in multiple cases shortly after sorivudine was approved in Japan.27PubMed Central. Sorivudine and 5-fluorouracil; a clinically significant drug-drug interaction due to inhibition of dihydropyrimidine dehydrogenase The mechanism is essentially the same as genetic DPD deficiency, but drug-induced and far more severe. This episode led to sorivudine’s withdrawal from the market and remains a case study in how a seemingly unrelated medication can turn a standard chemotherapy dose into a lethal one. The lesson extends beyond sorivudine: any drug or substance that inhibits DPD activity can potentially amplify 5-FU toxicity.
5-FU and Pregnancy
Cancer during pregnancy is rare but not unheard of, and when treatment involves 5-FU, the question of fetal safety arises. Exposure during the first trimester, when organs are forming, carries the highest risk and is generally avoided. However, clinical experience with inadvertent exposure or planned treatment using 5-FU-containing regimens (such as FOLFOX) during the second and third trimesters has been more reassuring. Reports have found no serious harm to fetal health from exposure during these later stages of pregnancy.28PubMed Central. Reproductive and developmental toxicities of 5-fluorouracil in model organisms and humans A broader review of chemotherapy during pregnancy concluded that fetal exposure to 5-FU, doxorubicin, and cyclophosphamide was not associated with serious short-term health concerns or cognitive abnormalities in offspring.29International Journal of Gynecological Cancer. Management of cancer during pregnancy and current evidence of obstetric, neonatal and pediatric outcome: a review article – Section: Chemotherapy These findings do not make second- or third-trimester use routine, but they provide some reassurance for the small number of pregnant patients who need fluoropyrimidine-based chemotherapy and cannot safely delay treatment.
Environmental Contamination in Hospitals
5-FU’s widespread use creates a less obvious concern: workplace exposure for the pharmacists who prepare it and the nurses who administer it. A workflow evaluation in a UK hospital found 5-FU contamination on isolator surfaces, benchtops, trays, and even on the outside of finished compounded products. Lower levels were also detected in the administration department on trays, trolley arms, and nurses’ gloves.30PubMed Central. Workflow evaluation of environmental contamination with hazardous drugs during compounding and administration in an UK hospital While the health consequences of chronic low-level skin exposure to 5-FU among healthcare workers are not fully understood, the finding underscores why closed-system transfer devices, proper gowning, and routine surface wipe testing are now standard practice in cancer pharmacy operations. For a drug that has been in use for over six decades, the science of safely handling it is still catching up.