Which of the following medications is used to treat congestion but should be avoided until after an illness has been treated?

The correct option is decongestants. These medications are effective at relieving nasal congestion, often associated with conditions like colds or allergies, by narrowing the blood vessels in the nasal passages, which reduces swelling and congestion. However, it is generally recommended to avoid using decongestants during an active illness, particularly when a bacterial infection is suspected. This is due to the fact that decongestants can mask symptoms, potentially leading to a delay in proper diagnosis and treatment of the underlying condition.

In contrast, antihistamines are typically used for allergic reactions and can be taken concurrently during illnesses that involve allergic symptoms. Antibiotics are specifically designed to combat bacterial infections and are essential in treating many illnesses, but they are not used for congestion. Expectorants serve to loosen mucus and facilitate its removal from the airways, assisting with productive coughs, but they do not address congestion in the same way. Hence, the use of decongestants should be carefully managed based on the patient's overall health status and the nature of their illness.