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Styes and chalazia (say “kuh-LAY-zee-uh”) are both conditions that can cause swelling of the eyelid.
A stye is an infection in the root of an eyelash. The infection causes a tender red lump on the edge of the eyelid. The infection can spread until the whole eyelid becomes red and inflamed. Styes usually break open, and a tiny amount of pus drains. They usually clear up on their own in about a week, but they sometimes need treatment with antibiotics.
A chalazion is a lump or cyst in the eyelid (chalazion is singular; chalazia is plural). It is caused by swelling and inflammation of deep oil glands inside the eyelid. Chalazia are usually not infected. They can take a few months to heal.
If a chalazion becomes more swollen and painful or does not go away, you may need to have it drained by your doctor.