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