Antihistamine tablets are a group of medications that prevent the level of histamine from rising in our body and reduce the symptoms this can cause. Antihistamines can be used to treat a range of conditions including allergic reactions, motion sickness and insomnia. However, the term "antihistamines" most commonly refers to medications that are used to treat hay fever.
If you are suffering from hay fever or allergic-type symptoms such as an itchy nose, itchy rash or bite on the skin or redness and minor swelling of the skin, then an antihistamine may help relieve these symptoms for you. Minor allergic reactions to products (washing detergent, creams, perfume), food or plant reactions (stinging nettles) can be safely treated with over-the-counter antihistamines and should get better in hours to days. Make sure to avoid whatever caused the reaction in future. If you find that you are getting regular allergic reactions and do not know the reason why, you should discuss with your doctor, and you may benefit from getting allergy testing.
Once-daily antihistamines are available to buy and contain loratadine or cetirizine. They are similar in effectiveness, but you may find one works better than the other. Both get to work within 1 to 3 hours, cetirizine perhaps slightly faster, and both peak in effectiveness after 8 to 12 hours, but last for at least 24 hours – loratadine possibly longer. At this dose, they are unlikely to make you drowsy.
Older antihistamines are more likely to cause drowsiness – this may be an advantage if symptoms are worse at night, but not if you need to operate heavy machinery or drive long distances. Those containing chlorphenamine (such as Piriton) or promethazine (such as Phenergan) are older types. They work for a shorter amount of time, typically 4 to 6 hours, so you might need to take them several times a day or just when the pollen count is higher, typically early mornings and evenings. Despite this, some people think they work better for their particular hay fever – it's a question of trial and error what works for you.
All options discussed above are available to buy over-the-counter. If they are ineffective, your doctor can prescribe fexofenadine, which is a stronger antihistamine.