The number of antihistamines you can safely take depends on the type and dosage of the antihistamine. For most common over-the-counter antihistamines like cetirizine and loratadine, the typical adult dose is 10 mg once daily. Taking more than the recommended dose, especially multiple doses at once, can be dangerous and lead to serious side effects. For example:

• Cetirizine: Usually 10 mg once a day for adults.
• Loratadine: Typically 10 mg once daily.
• Chlorphenamine: Usually taken every 4 to 6 hours, but maximum 6 tablets (each 4 mg) in 24 hours.
• Diphenhydramine: Up to 300 mg per day divided into multiple doses.

Taking more than one pill or multiple antihistamines simultaneously is generally not recommended unless directed by a doctor, as it raises the risk of overdose and side effects like extreme drowsiness, confusion, dizziness, and dry mouth. In summary, stick to the recommended dose on the medication's package or your doctor's instructions, which is usually one pill a day for many antihistamines. Always avoid combining antihistamines without medical advice to ensure safety.