What are antihistamines?

What are antihistamines?

Do you have hay fever or another type of allergy? If so, you’ve probably heard about antihistamines; indeed, you may already be taking them. These medicines have been helping people with allergy for over 75 years. Antihistamines can soothe symptoms from sniffles and sneezes to watery eyes and itchy rashes. Read on to find out how and why.

How do antihistamines work?

We need to look at histamine first. Histamine is one of the chemicals released by your body when it thinks it’s under attack.

When you come into contact with something your body perceives as a threat it sends a signal to mast cells lining your skin, respiratory tract (both upper and lower) and gut. These mast cells then release histamine.

Histamine is as part of an inflammatory response in which various fluids and white blood cells come to the site of the attack to help.

Histamine also gets the body to produce more mucus to flush out anything that doesn’t belong. In addition, it causes itching to make you scratch harmful material off your skin. All these reactions are designed to expel any intruder from your body as fast as possible.

But in allergy the “intruder” is a substance that is normally utterly harmless. Your over-eager immune system mistakenly identifies pollen, house dust mites, pet dander – whatever the trigger is – as a threat. It sends out a gush of histamine and you get allergy symptoms such as a rash, sneezing, coughing, runny nose and watery or itchy eyes.

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The name antihistamine is its actual job description: it’s blocking the effects of histamine. That’s a chemical in your body that causes annoying allergy symptoms.

That’s where antihistamine comes in. It blocks the effects of histamine. The name antihistamine is the drug’s actual job description.

Antihistamines: prevention or cure?

Antihistamines are among the most commonly used medicines for allergic conditions such as:

  • Sneezing, runny or blocked nose (allergic rhinitis)
  • Itchy, watery eyes (allergic conjunctivitis)
  • Hives (urticaria) or rashes
  • Allergic reactions to food

They do not cure allergies but for many people antihistamines can make living with allergy bearable.

Antihistamines can work as a short-term preventative measure, taken before you expect to be exposed to allergens. If pollen is your trigger, you may be able to predict when symptoms could start using a pollen forecast or calendar. You can talk to your health care provider about taking your antihistamines a couple of weeks before the season to build up protective antihistamine levels in your blood.

Life isn’t always as neat and tidy as this, of course. But you can use antihistamines after the reaction has started too. They’ll calm your allergy symptoms and block the release of more histamine. Remember that most antihistamines take about 1-2 hours to work.

What should I look out for?

You may know antihistamines best as tablets you swallow. But did you know they come as dissolvable tablets and in liquid form too? There are also nasal sprays, eye drops and creams. And the same antihistamine may be available in several different forms. You’ve maybe heard of non-drowsy antihistamines. What are they? Don’t worry, we’ll walk you through the different types.

Scientists have been developing new antihistamines since their discovery in the 1930s. They now tend to be divided into two types: first-generation and second-generation antihistamines. Which one you choose may depend on what time of day your symptoms are worst, so when you most need relief.

Some stronger antihistamines may need a prescription but you can buy many of them over the counter so ask the pharmacist for advice.

Did you Know...?

  • A medical study first described the symptoms of hay fever in 1819 but most doctors didn’t call it that until the 1860s.
  • Hay fever sufferers in the 19th century were advised to smoke tobacco as a cure, inhale ammonium chloride or even use opium.
  • The first substance with antihistamine properties was discovered in 1937 by scientist Daniel Bovet; he won a Nobel prize for this and other work.
  • The first antihistamine drug was ready for human consumption in 1942.
  • In 1947, an allergy patient stumbled on a happy side effect – antihistamine can help stop travel sickness.
  • Since then, some antihistamines have been found to help stomach problems, anxiety and insomnia too.

First-generation antihistamines

Older, first-generation antihistamines can make you feel sleepy. They usually carry labels warning against driving or operating heavy machinery after taking them. So you may prefer this type of antihistamine if hay fever is stopping you getting a good night’s sleep.

First-generation antihistamines include:

  • Chlorphenamine
  • Promethazine
  • Hydroxyzine

Second-generation antihistamines

Newer generation antihistamines are less likely to cause drowsiness. These are often labeled non-drowsy and used to relieve allergy symptoms throughout the day.

Second-generation antihistamines include:

  • Cetirizine
  • Loratadine
  • Acrivastine
  • Fexofenadine
  • Desloratadine

There is no definitively best antihistamine. They all essentially do the same thing but not necessarily as effectively for all symptoms or all people with allergy. You may find one particular antihistamine or way of taking it works better for you. Equally, not all of them are suitable for everyone. However, second-generation, non-drowsy antihistamines are usually preferable because they don’t make you sleepy.

Corticosteroid nasal spray

What are

Combination therapy

It’s not only antihistamines that have greatly evolved since their first discovery. So has the way we use them. People often partner these medicines with others. In fact, research has shown that for some people combination therapy, as it’s known, might be the way to go in treating symptoms of hay fever.

For instance, it’s possible to use a corticosteroid and an antihistamine nasal spray in a combined form. Decongestants are also used alongside antihistamines. But they should only be taken for short periods; for example, no more than 3 days in a row for decongestant nasal sprays. Their job is to tackle stuffy noses by shrinking swollen blood vessels in your sinuses, while antihistamines deal with the itchiness and sneezing.

Combination products already contain antihistamine so be careful not to exceed the total recommended dose if you’re using a separate antihistamine at the same time. You can ask your health care provider or pharmacist for advice on the specific products you’re taking.

Can children take antihistamines?

There are antihistamines for children over the age of 1. Some types are available over the counter. But it’s best to seek advice from your healthcare provider first about which might be most suitable for your child. And keep a close eye on them to make sure there are no unwanted side effects.

What are the side effects of antihistamines?

Like any medicine, antihistamines can cause side effects. And also like any medicine, they come with an information leaflet. Make sure to read it before you start tackling your allergy symptoms. The side effects of the different generations of antihistamines are distinctive.

Side effects to old, first-generation antihistamines could be:

  • Drowsiness, poor co-ordination and reduced reaction speed and judgement. Hence the risk in driving or using machinery while taking them.
  • Dry mouth
  • Blurred vision
  • Trouble peeing

Side effects of second-generation antihistamines could be:

  • Headache
  • Dry mouth
  • Nausea
  • Drowsiness, but it is much less common for these newer antihistamines to make you sleepy.

Hay fever symptoms?

A person wearing a gas mask to avoid triggering hay fever symptoms

Be safe, be sensible when taking antihistamines with other medicines

Always speak to your health care provider or pharmacist if you’re already taking other medicines and consider using antihistamines. Antihistamines can interfere with how other drugs work, and vice versa. You’ll find all these details on the information leaflet that comes with the antihistamine.

As a rule, it’s not a good idea to drink alcohol, especially if you’re taking the older type of antihistamine.

There is also a risk of unintentionally doubling up on your dosage by using another medicine containing antihistamine at the same time. We’ve already talked about decongestants above.

How to take antihistamines

Interactions with other drugs are not the only reason to read the information leaflet in your antihistamine packet.

How you use these medicines varies from type to type:

  • If it’s an oral antihistamine, does it have to be taken with water or food.
  • With eye drops or a nasal spray, how exactly do you use it so that it does the job properly.
  • What’s the right dose – it may depend on things like your age and weight.
  • How many times a day can you take this particular antihistamine, and when.
  • How long can you use it for – some are only recommended for a few days at a time.

Antihistamines and real life

You might ask yourself, “What’s the best medicine for my hay fever?” Well, there is no definitive answer: What’s right for one person, might not be right for another. Your health care provider or pharmacist will be able to help you if you’re in doubt.

For many people antihistamines can be a good place to start. And you can help them to help you by managing your exposure to your triggers. For example, if you have hay fever, it’s smart to check the pollen forecast so you can plan your time outdoors around when the levels are low. In our allergy app, you can also keep track of when you’ve taken your allergy meds, log your symptoms and much more.

What if regular allergy meds are not enough?

Sometimes antihistamines and other allergy relief meds can’t keep allergy symptoms in check. In which case your health care provider might discuss allergy immunotherapy with you. This treatment tackles the underlying cause of your allergy.

Repeated tiny doses of an allergen, either as injections or as tablets under your tongue, desensitize your immune system. Your body gets used to that trigger and in time learns not to react. You should have fewer symptoms and need less allergy medication.

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