Drugs degrade on storage. Regulations set the maximum amount of degradation at 5% unless other factors intervene, which would be very unlikely for an OTC/P medicine. The shelf life/Best before date is determined from the date of manufacture. The 5% limit is to ensure consistency of efficacy/performance, not toxicity. The shelf life represents the 95% lower confidence interval (or less) that the maximum degradation could have occurred under the stated storage condition over that time period. Going past this date would possibly mean that the product is less efficacious. It won't be dangerous because it either wouldn't be licensed if it was, or would be restricted to use in a medical setting.