Generally speaking, you shouldn’t breastfeed when undergoing systemic chemotherapy with 5FU. However, if your treatment is intermittent rather than continuous, you may be able to breastfeed. Always consult with your doctor first.
If you’re using 5FU topically, you can still breastfeed as long the 5FU isn’t being applied to your breast or nipple. Also take care to prevent your infant’s skin from coming into contact with any 5FU cream on your body.