Fluorouracil (5-FU)

How it works

Its metabolites inhibit thymidylate synthase and are incorporated into RNA and DNA, disrupting nucleotide synthesis and impairing DNA replication and cell division.

Prescribing in practice

• Patients with dihydropyrimidine dehydrogenase (DPD) deficiency are at risk of severe, life-threatening toxicity, so DPD testing is recommended before systemic treatment.
• Administer systemic fluorouracil only under specialist oncology supervision, with caution and dose modification in hepatic and renal impairment.
• Severe mucositis, diarrhoea, myelosuppression and cardiotoxicity (including coronary vasospasm) can occur and may necessitate stopping treatment.

Monitoring

Monitor full blood count, mucosal and gastrointestinal toxicity, and cardiac symptoms, with DPD status assessed before starting.

Counselling the patient

• Report severe mouth ulcers, diarrhoea, or any chest pain promptly.
• Report signs of infection or unusual bruising or bleeding without delay.
• Use sun protection, as your skin may become more sensitive to sunlight.

Evidence & guidelines

MHRA advice recommends DPD deficiency testing before fluorouracil, and the drug remains a backbone of evidence-based regimens such as those used in colorectal cancer.