You can give your dog several types of antihistamines, but it's important to use the correct kind and dosage, and ideally only under veterinary guidance. Common antihistamines safe for dogs include:

• Diphenhydramine (Benadryl): Often used for allergies, insect bites, and motion sickness. Typical dose is about 1 mg per pound of body weight every 8-12 hours.
• Cetirizine (Zyrtec): Used for itching and inflammation, with typical dosing around 1 mg per kg or 10-20 mg per dog every 12-24 hours.
• Chlorpheniramine: Dose varies by weight; for dogs under 20 kg, 4 mg every 8 hours, and for dogs over 20 kg, 8 mg or 0.25-0.5 mg/kg every 8 hours.
• Hydroxyzine and Clemastine are also used but often require a prescription.

Each antihistamine may affect individual dogs differently, so it is recommended to try one for 2-3 weeks to see effectiveness before moving to another. Avoid products with xylitol, a sugar substitute toxic to dogs. Always consult your vet before giving any antihistamine to confirm the right type and dosage for your dog’s specific condition and health.