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Doctors often recommend drinking extra fluids to patients with respiratory infections. Theoretical benefits for this advice are replacing insensible fluid losses from fever and respiratory tract evaporation, correcting dehydration from reduced intake, and reducing the viscosity of mucus. However there are theoretical reasons for increased fluid intake to cause harm. Giving extra fluids while antidiuretic hormone secretion is increased (reported in some lower respiratory tract infections) may theoretically lead to hyponatraemia and fluid overload. Despite a systematic review to determine whether recommending increased fluids was beneficial or harmful no randomised controlled trials comparing increased and restricted fluid regimens in patients with respiratory infections were found. Without this evidence we should be cautious about universally recommending increased fluids to patients, especially those with infections of the lower respiratory tract.