antihistamine tablets usually start to work within 30 minutes and are likely to be most effective within 1 to 2 hours. For allergies such as hay fever, antihistamines are more effective when taken regularly as a prevention, before symptoms occur, rather than only when you have symptoms.
Antihistamines are medicines often used to relieve symptoms of allergies, such as hay fever, hives, conjunctivitis and reactions to insect bites or stings. They're also sometimes used to prevent motion sickness, to treat feeling sick (nausea) or being sick (vomiting), and as a short-term treatment for insomnia.
Of these, cetirizine works the fastest but has a higher chance of making you feel drowsy. Fexofenadine is the least drowsy option. We can't say for certain which non-drowsy antihistamine is best for allergy symptoms.
In general, an oral antihistamine tablet will start to work within 30 minutes and reach its maximum effect in about 2 hours. You will know when an antihistamine starts to work when it starts to relieve allergy symptoms, such as congestion, runny nose, itchy eyes or skin rash.
The release of histamine within the skin seems to be the trigger for swelling in most people. Antihistamines block this effect. Since they take one to two hours to reduce symptoms, it is often better to take them regularly, rather than as needed.
As well as taking a non-drowsy antihistamine during the day (such as cetirizine or loratadine), your doctor may advise that you take a drowsy antihistamine at night time if the itching is making it difficult to sleep. Do not take 2 antihistamines together unless recommended by your doctor.
If a healthy adult takes only a slightly higher dose of antihistamine, such as accidentally taking two pills instead of one, their symptoms may not be serious, or they may not have any symptoms. However, a larger overdose, especially in children or older adults, may cause serious symptoms.
Do NOT "double-up" on a dose. Do NOT take a dose sooner than you're supposed to. Do NOT take two different antihistamines at the same time. Instead, talk to your doctor or pharmacist if you have trouble finding a drug that works well for your allergy symptoms.
What Antihistamine is Right for you? You can buy many different brands and forms of antihistamines without a prescription. Some work for only 4 to 6 hours, while others last for 12 to 24 hours. Some are combined with a decongestant, a drug that dries up your nasal passages.
While antihistamines may help you to fall asleep, overall sleep quality is usually not very good, he said. Sometimes, they even have the opposite affect and cause hyperactivity, especially in children. “Using Benadryl or any antihistamine for sleep has no long term benefit,” Alapat said.
There is no substitute for epinephrine, which is the only first-line treatment for anaphylaxis. Neither antihistamines nor glucocorticoids work as quickly as epinephrine, and neither can effectively treat the severe symptoms associated with anaphylaxis.
This will depend on the type of reaction, how easy it is to avoid the trigger, and whether or not the person seeks treatment. Hives often go away within a few days or weeks. Treatment may speed the end of the rash, but some reactions can take up to a month to disappear, even with treatment.
An allergic reaction can last for varying amounts of time. An allergic reaction can typically last from a few hours to a few weeks. If you have prolonged exposure to your allergen, your allergic reaction will usually last longer.
But with this acute itching, a different type of cell in the bloodstream transmits itch signals to the nerves. Those cells produce too much of another non-histamine substance that triggers itch; therefore, antihistamines don't work in response to such signals.
Antihistamines can stop working because your allergies might be getting worse or you might have immune system changes due to aging. You also might not be taking the medication as directed. If you take an antihistamine after being exposed to an allergy trigger, it won't work as well.
You shouldn't take prescription antihistamines if you also are taking certain other prescription medicines. These include erythromycin (an antibiotic), or certain medicines used to treat fungal infections. These include itraconazole or ketoconazole.
Cetirizine is the most potent antihistamine available and has been subjected to more clinical study than any other.
Some antihistamines are taken every 4 to 6 hours, and some are longer-lasting antihistamines that are taken every 12 to 24 hours. Consult the drug label and ask your health care provider how long and how often you should take an antihistamine.
Sedating antihistamines
As well as sedation, overdose often presents with anticholinergic symptoms (see Anticholinergic Syndrome). At higher doses, they can also cause sodium channel blockade with subsequent cardiovascular effects including QT prolongations and Torsades de Pointes.
Official answer. No, the maximum recommended daily dose of loratadine is 10mg once daily.
It's not a great idea, but an isolated incident of taking two Zyrtec is unlikely to have major health effects. In fact, some doctors double the dose of Zyrtec for their patients with severe hives if a single pill isn't working.