Antihistamines: definition, types, recommendations & tolerability profile

What are allergies and what roles does histamine play? 

An allergy is when the immune system mistakenly treats harmless substances as a threat [2.b]. The immune system’s response may be experienced as localised itching or a rash. However, in severe cases, the response can cause anaphylaxis (anaphylactic shock). Anaphylaxis can lead to respiratory obstruction, inflammation and blockage of the airways. It can be fatal. [8]

When the allergen comes into contact with the body, it triggers an immune response. In the body, histamine is stored in mast cells. When an allergen triggers these cells, histamine is released [9]. Histamine causes the typical symptoms of allergic reactions, e.g. swelling, inflammation and itching [9]. 

Common symptoms of allergic reactions include [2.e, 6,7]: 

• nasal symptoms:
  • sneezing [2.e]
  • runny nose [2.e]
  • itchy throat [2.e]
  • itchy nose [2.e]
     
• skin symptoms:
  • itching [2.e]
  • itchy throat [2.e]
  • itchy eyes [2.e]
  • urticaria [2.e]
  • skin rashes and hives [2.e].
     
• eyes symptoms
  • watery eyes [2.e]

The most common causes of allergic reactions are [6,7]: 

• pollen  [2.f]
• dust mites  [2.f]
• food, especially peanuts, tree nuts, milk, eggs and shellfish [2.f]
• insect stings or bites [2.f]
• some medicines [2.f]
• mould 
• pet hair [2.f]
• air pollution 

Severe allergic reactions 

In rare cases, the allergic reaction can be severe and life-threatening. Such reactions require immediate medical attention and urgent treatment with an AAI (Adrenaline Auto Injector). [10]

Types of antihistamine 

There are two main types of antihistamines [2.g]: 

• first-generation antihistamines (which cause drowsiness), and 
• second-generation antihistamines (which do not cause drowsiness) 

First generation 

These oral antihistamines can easily cross the blood-brain barrier [2.h]. H1 receptors in the central nervous system regulate the sleep-wake cycle [2.i]. These medicines bind themselves to the central H1 receptors, which causes drowsiness and may also impair cognitive and motor functions [2.j]. 

First-generation antihistamines include: 

• Dimetindene [11]
• chlorpheniramine [12]
• clemastine [2.k]
• doxylamine [2.k]
• hydroxyzine [2.k]
• promethazine [2.k]
• triprolidine [2.k]

Second generation 

These types of antihistamines do not cross the blood-brain barrier very easily [2.l]. They therefore do not have such significant effects on the CNS as first-generation types. They also tend to be safer and more potent than first-generation antihistamines [2.m]. 

Second-generation antihistamines include: 

• bilastine [2.n]
• desloratadine [2.n]
• loratadine [2.n]
• fexofenadine [2.n]
• cetirizine [2.n]

When to take antihistamines 

As with all over-the-counter and prescription medicines, antihistamines should be taken as instructed by the pharmacist or doctor, or in accordance with the package leaflet [1.d]. Pay attention to factors such as: 

• whether to take the medicine with food or water, or on an empty stomach 
• how much to take 
• how often and at what time of day to take it 
• how long to take it for 
• what to do if you miss a dose, or take too much 

Different antihistamines may be taken in different ways, depending on the cause of your symptoms. For instance, if you suffer from  hay fever ( allergic rhinitis ), you will only need to take medication during allergy season. Alternatively, if you can remove the allergen from your environment (e.g. pet allergy), you will only need to take antihistamines for a short period, until your symptoms subside. 

Second-generation H1 antihistamines are generally well tolerated because of their decreased ability to cross the blood-brain barrier compared with first-generation agents. Second-generation H1 antihistamines have a lower potential to cause CNS effects, in particular sedation. [13]  

Second-generation H₁-antihistamines are more adapted than first generation antihistamines for the use in children with AR and CIU due to better safety profiles. [14] 

Patients should avoid driving or other similar activities until they can gauge how the product, they are taking affects their motor skills and cognitive functions [3.d]. 

Drinking alcohol may intensify any side effects, so patients should avoid it while taking antihistamines [3].  

Drug free alternatives 

For those for whom antihistamines are contraindicated or who do not wish to take them, the following drug free remedies might be considered [15]: 

Saline sprays. These over-the-counter saltwater sprays can flush out the nasal passages. They help clear out pollen and other irritants.

Supplements. There's evidence that some supplements help nasal allergies. Butterbur is one of the most promising and well-researched. Studies show that butterbur -- specifically a butterbur extract called Ze 339 -- works as well as some allergy drugs.

Check with your doctor before you start using any supplements regularly, especially if you take daily medication or have any health conditions.

Probiotics [16] are microorganisms that might offer health benefits by helping the body maintain a healthful balance of gut bacteria. Probiotics may boost a person’s immune system, which can help the body fight off allergies.

The evidence for probiotics is mixed and some probiotics may help while others may not.

Quercetin [16] is an antioxidant flavonoid found in many plants and foods. Research suggests that adding quercetin to the diet may help to relieve allergy symptoms. Research reports that quercetin can have anti-allergic and antihistamine properties, however the evidence for its effectiveness is mixed.

Finding the right antihistamine 

Over-the-counter antihistamines can be effective in managing allergies. The key is to balance the benefit and risk in a way that works for you. Medical experts suggest: 

• Choosing a long-acting and non-sedating antihistamine, e.g.  loratadine, desloratadine, cetirizine, fexofendadine and levocertirizine.  [17]

If, despite taking an over-the-counter product, your symptoms are persistent, discuss the matter with your doctor or pharmacist [1.f].