People don’t drink spoiled milk, but they do wear contact lenses past their prime. According to a new survey, 40 percent of contact users don’t replace their lenses, ignoring manufacturer recommendations.

Researchers at the University of Waterloo in Canada surveyed 1,654 U.S. adults who wear contact lenses. Among the participants, 45 percent wore silicone hydrogel lenses requiring replacement every 14 days; 39 percent wore silicone hydrogel lenses requiring replacement monthly; and 16 percent wore disposable lenses requiring replacement each day.

The findings showed that, overall, 40 percent of the participants kept their lenses beyond the suggested date. This included 59 percent of patients with two-week lens replacement limits, 29 percent with one-month expiration dates, and 15 percent of those with daily lens replacement limits.

Why were they ignoring their lens expiration dates? Patients with two-week or one-month replacement dates told researchers that they simply forgot. Those wearing daily lenses just wanted to save money.

Learn how to keep your eyes healthy here.