Small to large amounts of drainage and different types of drainage may be caused by irritation, a foreign body, or infection. The drainage often collects on the eyelashes and dries, causing the lashes to become matted together. The eyelids may also swell during the night. Once the matted drainage is removed, the swelling usually decreases.

Eye infections are usually caused by a virus or a bacteria. In some parts of the world, eye infections may also be caused by fungus, parasites, or other organisms. Drainage occurs with almost every type of eye infection, such as pinkeye (conjunctivitis).