The pupil is the black center part of the eye that enlarges when subjected to poor lightening to help you see better. Pupils may also dilate as the result of recreational and medical drug use, as well as more serious health conditions -- including poisonings. If you don't want others to notice your dilated pupils, there are a few things you can do to cover them up.
Wear a hat or baseball cap. Either cap will partially shade or hide your eyes, making dilation less noticeable.
Stay in a well-lit area to keep pupils from dilating further. Pupils can dilate as a reaction to dim lighting, so avoid poorly-lit areas, until your pupils recede.
Wear a large pair of dark sunglasses. Sunglasses are arguably the best choice for covering up dilated pupils, as they completely cover the eyes. Avoid lighter pairs, where the eyes are noticeable through the lenses.