If you have allergies and find that the medication is no longer effective after 12 hours, it is important to consult a healthcare professional before considering taking a second dose within a 24-hour period. It is not recommended to exceed the maximum dose suggested by the manufacturer without medical advice. In such cases, it may be necessary to consider modifying the treatment or adding a different medication. Allergy symptoms can vary, so it is important to discuss your specific symptoms and possible treatment options with a healthcare provider for proper guidance.