Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed colorectal cancer

- Advanced or metastatic disease

- Scheduled to receive first- or second-line therapy for metastatic disease

- No cerebral metastases or symptomatic or uncontrolled meningeal disease

PATIENT CHARACTERISTICS:

- WHO performance status 0-2

- ANC ≥ 2,000/mm^3

- Platelet count ≥ 100,000/mm^3

- Total bilirubin ≤ 2 times upper limit of normal (ULN)

- AST and ALT ≤ 3 times ULN

- Alkaline phosphatase ≤ 5 times ULN

- Not pregnant or nursing

- Fertile patients must use effective contraception

- No intestinal blockage

- No complete dihydropyrimidine dehydrogenase deficiency

- No chronic inflammatory disease of the colon

- No other cancer except for nonmelanoma skin cancer or curatively treated carcinoma of
the cervix or breast

- No other severe condition, or condition that is likely to worsen, including any of
the following:

- Unstable heart disease

- Myocardial infarction within the past 6 months

- Active uncontrolled infection

- No contraindication to atropine

PRIOR CONCURRENT THERAPY:

- See Disease Characteristics

- Recovered from prior anticancer therapy

- More than 4 weeks since prior and no other concurrent investigational therapy

- Prior adjuvant chemotherapy allowed

- No prior fluorouracil or irinotecan hydrochloride

- No other concurrent anticancer therapy