This is a fixed combination of three oral compounds, tegafur, gimeracil, and oteracil (potassium), known together as S-1, used as chemotherapy for advanced gastric cancer. Tegafur is a prodrug of the well-established chemotherapy drug 5-fluorouracil (5-FU), and the other two components are added specifically to improve its effectiveness and reduce certain side effects. It is a prescription-only oncology medicine.

• Therapeutic Class: Anti-cancer (oncology) chemotherapy, oral fluoropyrimidine.

• Pharmacological Class: 5-fluorouracil prodrug (tegafur) combined with a dihydropyrimidine dehydrogenase inhibitor (gimeracil) and a gastrointestinal-toxicity-reducing agent (oteracil).

• Drug Family: Fluoropyrimidine chemotherapy combinations.

• ATC Classification: L01BC53 (Tegafur, combinations).

Approved: Advanced gastric cancer, in combination with cisplatin, in regions where approved (widely used in Japan and parts of Asia and Europe; approval status varies by country).

Commonly used for: Patients with unresectable or metastatic gastric cancer, as a first-line chemotherapy option, and as adjuvant (after-surgery) chemotherapy in some countries following curative gastric cancer resection.

Off-label use (only under medical guidance): Sometimes used in other gastrointestinal cancers, such as esophageal cancer, particularly in combination with radiotherapy, under specialist oncology guidance.

● Oral capsules: commonly available in strengths reflecting the fixed tegafur:gimeracil:oteracil molar ratio of 1:0.4:1, such as capsules delivering 20 mg or 25 mg of tegafur (with proportional gimeracil and oteracil), for weight/body-surface-area-based dosing.

Tegafur is converted in the body into 5-fluorouracil (5-FU), a chemotherapy drug that interferes with cancer cells' ability to make and repair DNA, causing cell death. Gimeracil blocks an enzyme (dihydropyrimidine dehydrogenase) that would otherwise break down 5-FU too quickly, helping maintain more consistent, effective drug levels. Oteracil concentrates mainly in the gut and blocks a different enzyme there, reducing the gastrointestinal side effects that 5-FU-based treatment can otherwise cause.

• Standard dose (with cisplatin, advanced gastric cancer): Commonly dosed at around 40-60 mg/m2 (of the combination) orally twice daily, taken on specific days of a treatment cycle in combination with cisplatin, per the specific regional protocol used.

• Condition-wise dose (adjuvant, post-surgery): Regimens such as 80-120 mg/body/day (total daily dose based on body surface area) for 4 weeks followed by 2 weeks off, repeated for a defined number of cycles, have been used in adjuvant settings.

• Route: Oral, in divided doses (commonly twice daily), on a cyclical schedule with treatment and rest periods.

• Missed dose: Managed by the treating oncology team; not typically self-adjusted by the patient without guidance.

• Overdose: May worsen known chemotherapy-related side effects such as severe diarrhoea, mouth sores, or low blood counts; requires urgent medical attention.

• Common: Nausea, diarrhoea, loss of appetite (anorexia), fatigue, and mouth sores (stomatitis).

• Serious: Low blood counts (neutropenia, leukopenia), elevated liver enzymes/bilirubin, and, when combined with cisplatin, added risks from the cisplatin component (kidney effects, nausea, hearing changes).

• Emergency symptoms: Signs of serious infection (fever, chills) linked to low white cell counts, severe or persistent diarrhoea, yellowing of the skin/eyes, or significant unexplained bleeding or bruising.

• Recovery advice: Anorexia and mild digestive symptoms are common early in treatment; regular blood count and liver function monitoring help the oncology team adjust doses before more serious effects develop.

• Pregnancy: Not recommended; fluoropyrimidine-based chemotherapy can cause fetal harm.

• Breastfeeding: Not recommended during treatment.

• Liver disease: Used with caution; liver function is monitored, particularly given elevated bilirubin/liver enzymes seen in trials.

• Kidney disease: Requires dose adjustment based on kidney function, since the metabolism and clearance of this combination can be affected by reduced kidney function.

• Elderly: Used with routine monitoring; may require closer observation for tolerability.

• Children: Not established for use in children; a medicine for adult gastric cancer treatment.

● Other 5-FU-based chemotherapy or fluoropyrimidines (e.g., capecitabine): not combined with this combination, given overlapping mechanism and toxicity.

● Phenytoin: 5-FU-based regimens, including this combination, can increase phenytoin levels, requiring closer monitoring.

● Warfarin and other anticoagulants: fluoropyrimidine chemotherapy can enhance the effect of warfarin, increasing bleeding risk; closer monitoring of clotting levels is recommended.

● Other bone-marrow-suppressing drugs or radiotherapy: may increase the risk of low blood counts when combined.

● Complete blood counts regularly throughout treatment cycles.

● Liver function tests (including bilirubin) periodically.

● Kidney function tests, particularly given the need for dose adjustment based on renal function.

● Nutritional status and symptom monitoring, given the common gastrointestinal side effects.

Capecitabine (another oral fluoropyrimidine prodrug), infusional 5-fluorouracil, and other gastric cancer chemotherapy regimens depending on stage and prior treatment.

Most commonly used in combination with cisplatin for advanced gastric cancer; also studied and used with other chemotherapy or targeted agents, and alongside radiotherapy in certain gastrointestinal cancer protocols, depending on regional practice.

● Gastric Cancer Medicines

● Oral Fluoropyrimidine Chemotherapy

● Gastrointestinal Cancer Treatments

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1.  https://www.dovepress.com/tegafurgimeraciloteracil-s-1-approved-for-the-treatment-of-advanced-ga-peer-reviewed-fulltext-article-OTT

2. https://drugs.ncats.io/drug/UA8SE1325T

The information on this page is for educational purposes only and is not a substitute for medical advice. Please consult your oncologist to know if this treatment is appropriate for your condition.