Tears are produced by the main lacrimal gland, which is located in the superotemporal orbit, and by accessory lacrimal glands within the eyelids. Tears leave the ocular surface via the puncta within the medial aspect of the eyelids and pass from the canaliculi to the nasolacrimal duct and into the nose. Tearing occurs when relatively more tears are produced than can be drained through the lacrimal drainage system.

Primary hypersecretion of tears has not been identified as a cause of tearing, but secondary hypersecretion, or increased tear production in response to stimulation, can occur as a result of irritation to the ocular surface (e.g., corneal abrasion), reflex tearing in the setting of severe dry eye, inflammation within the eye, aberrant regeneration after Bell palsy, and ocular allergy. Even when the outflow pathway is patent, the relative increase in tear production will result in a watery eye.

Insufficient or impeded drainage in the setting of a normal rate of tear production may also result in tearing. Abnormalities in this category may include lid malposition (e.g., ectropion), punctual stenosis, canalicular stenosis, and nasolacrimal duct obstruction. Most patients with tearing have impeded outflow as the cause rather than overproduction of tears.

Tear film movement may be obstructed by eyelid-margin lesions or conjunctival redundancy or folds. The pumping function of the lid may be impaired by seventh nerve palsy or conditions stiffening the lids, such as scars or scleroderma, or by laxity of the lids from aging. Thyroid eye disease may lead to tearing from corneal irritation due to proptosis and lid malposition. The puncta must be properly positioned. Ectropion prevents tears from gaining access to the canaliculus. Senile ectropion is the most common cause in the elderly and is characterized by a sagging lower lid. The punctum or the canaliculus may be occluded congenitally, by chemical or thermal injury, or by neoplasms. In addition, canalicular infections may cause occlusion. The most common of these are Actinomyces israelii (Streptothrix) and Candida. Finally, obstruction of the lacrimal sac and nasolacrimal duct may be idiopathic, congenital, or caused by neoplasms (particularly lymphoma), ethmoiditis, and turbinate disease. The more distal the obstruction, the more likely it is that the epiphora will be accompanied by purulent discharge or dacryocystitis, as the stagnant tears become infected.