Hi. Thanks for the clarification. Since you're not ingesting the castor oil, then there's probably no harm in having those packs while you're undergoing chemotherapy.
This castor oil would not be taken internally, but rather lighly soaked on wool flannel and placed on the stomach with a heating pad. Would that make a difference?
Hi. As a rule, it would be better for you not to take any unnecessary medication (whether synthetic or herbal) while undergoing chemotherapy. Chemotherapy places your body in a precarious state and any additional medication can push you to have an increase in severity of adverse events. If you need to have castor oil, maybe you can have it a week after your chemotherapy has been given.