Do not start or increase a diuretic solely for swelling that is clearly worse in one leg.

Heart-failure fluid retention usually causes both legs to swell. New or increasing one-sided leg edema can be a deep-vein thrombosis (DVT), venous insufficiency, cellulitis, injury, or lymphatic obstruction. A DVT needs prompt assessment and may require an urgent leg-vein ultrasound and anticoagulation, not a diuretic. Medical references specifically list DVT and chronic venous insufficiency among causes of unilateral leg swelling.

He should be assessed today, especially if this difference is new or worsening. Go to emergency care now if he has any of these:

Do not use someone else’s furosemide/Lasix or change an existing dose without the prescriber. Diuretics used for heart failure, commonly loop diuretics such as furosemide, can help general congestion, but require a clinician to assess blood pressure, kidney function, sodium, potassium, weight, and current medications. They can cause dehydration, kidney injury, low potassium/sodium, and low blood pressure, while leaving a DVT untreated.

Until evaluated: keep the leg gently mobile if comfortable, avoid massage, and do not begin compression stockings until DVT and significant arterial disease have been considered. If he already has a heart-failure action plan with a clinician-directed diuretic adjustment, contact that clinician urgently and report the unilateral swelling rather than independently changing the dose.