What is the name for a substance that you take when you are ill?

The term for a substance that you typically take when you are ill is "medicine." Medicine is specifically formulated to prevent, cure, or alleviate the symptoms of diseases and illnesses. This category includes a wide variety of products such as prescription drugs, over-the-counter medications, and even herbal remedies that are taken to help restore health or improve well-being during sickness.

In contrast, a treatment refers more broadly to any procedure or method used to manage a disease or condition, which could include non-medicated interventions such as therapy or lifestyle modifications. A supplement is primarily intended to add nutritional value and usually does not directly treat illnesses. A vaccine is designed to provide immunity against specific diseases but is not taken during illness; it is administered beforehand to prevent infection from certain pathogens. Therefore, "medicine" is the most precise term to indicate the substance taken when a person is suffering from an illness.