I have uneven pupil dilation. Is this harmful or harmless?

See an eye doctor. It can be normal, especially if it has always been present. There are certain conditions that can lead to one pupil dilating more (or less) than the other. If the pupil difference is new, it should be evaluated by an eye doctor soon. The condition itself is usually harmless, but the underlying cause needs to be addressed.
See an eye doctor. Before the staff or doctors dilate your eyes explain to them that your visit is about your pupil differences. The exam for pupils should include documentation of size in dark and lit conditions of each eye. Also how the fast each pupil reacts to light and how they compare to each other. Color vision of each eye. The doctor will look for diseases that affect nerve. Likely normal variant.
This Depends. This depends on the degree to which the two pupils are uneven in diameter. Uneven pupil size is known medically as “anisocoria”. Anisocoria may result from many conditions, some harmless and some life threatening. In general, we allow for 1 to 2 millimeters of physiologic (normal) difference in pupil size. However, if the discrepancy in pupillary diameters exceeds this, a significant underlying problem may exist, especially if new in onset. The challenge for the clinician is to determine which pupil is the abnormal one: the larger pupil or the smaller pupil. If the problem lies with the larger pupil, a variety of conditions may be accountable ranging from benign to serious. A common benign cause of pupil dilation is the inadvertent introduction of a dilating compound into the eye. For example, a patient using a nasal degongestant/vasoconstrictor spray containing the adrenergic agent neosynephrine may inadvertently spray some into the eye, resulting in pupil dilation. Likewise, a patient using a scopolamine patch for sea-sickness may inadvertently get some of this anticholinergic agent compound onto his or her hands and rub the eye, resulting in pupil dilation. Also, people who tend to plants may inadvertently get some sap on their hands and rub the eyes. The sap of some plants contain belladonna alkaloids which are potent anticholinergics which may result in pupil dilation. More serious causes of pupil dilation include trauma, resulting in pupillary margin tears, a condition called adie’s tonic pupil and, finally, of greatest concern, a third nerve palsy. A third nerve palsy with a dilated pupil often results from a cerebral artery aneurysm and is a medical emergency as it may lead to intracranial hemorrhage and death. Other signs of a third nerve palsy are new onset double vision, a droopy eyelid and the restriction of eye movement such that it moves “down and out”. If these signs are present, especially if there are attendant headaches, the patient must go to the emergency room immediately for brain imaging and possible neurosurgical intervention. If the problem is with the smaller pupil, then the list of possible conditions includes, most seriously, a horners syndrome, which results from interruption of the sympathetic nerve chain as it travels from brain, through thorax, back up to eye via internal carotid artery. Any compressive lesion along this pathway may interrupt nerve impulses directing pupil dilation, resulting in pupil constriction or “miosis”. Worrisome lesions include certain types of lung cancer and carotid artery dissection which is a medical emergency. Other signs of horners syndrome include droopy eyelid on the affected side, and decreased sweat production of the facial skin on the side of the problem.