Answer
Antihistamine effects are generally apparent within a few hours, although individual response varies.
Sources for this answer
- Well Pharmaceuticals One-a-Day Allergy Relief 10mg tablets — Current selected SmPCemc · Summary of Product Characteristics
- Clarityn Allergy 1mg/ml Syrup — Current selected SmPCemc · Summary of Product Characteristics
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.