The timing of chest pain symptoms from 5FU chemotherapy depends on the type of chemotherapy (if it is given as an IV or as a pill) and how it is given (in one dose or continuous infusion).
IV infusion (5FU): When given in one dose (as a bolus), chest pain can occur during the infusion or a few hours afterward.
The continuous IV infusion is given over two to four days with a pump that you may take home. When given this way, the chemotherapy may reach certain parts of the body more slowly. Symptoms can occur between 12-72 hours after starting the infusion. With more cycles of continuous infusions, symptoms may begin earlier in the cycle, become more intense, and last longer.
Oral pill (capecitabine): The pill version, capecitabine, is processed by the body more slowly. As a result, the symptoms are like those experienced with continuous infusion. Side effects may be less intense and start 12-72 hours after starting the medicine.
People who get heart symptoms or other problems from 5FU chemotherapy usually feel the side effects during the first or second cycles of treatment. Most people go through these chemotherapy regimens without heart complications.
If you don't have symptoms during your first or second cycles, then it is unlikely you will have heart side effects later.