Is it a sin for a Catholic to stop chemotherapy?

Not in itself, and the teaching here is more permissive than most people expect.

Catholic teaching distinguishes two categories. Proportionate means are those that offer a reasonable hope of benefit and do not entail an excessive burden or expense, and a person has an obligation to use them. Disproportionate means are those that do not offer a reasonable hope of benefit, or that entail excessive burden or expense, and the Ethical and Religious Directives say plainly that "a person may forgo extraordinary/disproportionate means of preserving life."

The seventh edition, approved by the US bishops in November 2025, is explicit about who decides: "The final determination as to what constitutes a proportionate benefit and what constitutes an excessive burden belongs to the patient (or the patient's surrogate) and should be informed by professional medical advice."

Cost is a legitimate part of that judgment. The Directives name excessive expense on the patient, the family, or the community.

John Paul II addressed the fear behind the question directly, writing that to forgo extraordinary or disproportionate means "is not the equivalent of suicide or euthanasia; it rather expresses acceptance of the human condition in the face of death."

Whether a particular treatment is proportionate for you is a decision for you with your care team, and a hospital ethics consultation can help.