Antihistamines are available in two main types: first-generation and second-generation. Antihistamines are available in two main durations of action: short-acting and long-acting. Antihistamines are available in two main ways to purchase: over the counter and by prescription.
Generations:
First-generation antihistamines: These are older antihistamines, such as diphenhydramine (Benadryl) and chlorpheniramine (Chlor-Trimeton). They are effective but can cause drowsiness as a side effect.
Second-generation antihistamines: These are newer medications, such as cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra). They are less likely to cause drowsiness and are often preferred for daytime use.
Durations:
Short-acting antihistamines: These may need to be taken multiple times a day.
Long-acting antihistamines: These provide relief for 24 hours.
Purchases:
Over the counter (OTC): This means you can buy them without a prescription.
By prescription: This requires a doctor or equivalent, and is for more-severe conditions or specific medical purposes.