How Do Antihistamines Work?

Your body has a great security alert system that is good at detecting threats like pollen or insect bites. When it detects these intruders, the body releases something called histamine. This is a chemical that acts like an alarm bell to trigger reactions in the body to protect you. It is this that causes the very uncomfortable itchy eyes, sneezing, or skin rashes. Sometimes this security system is overly sensitive and produces a larger response than necessary. Antihistamine medications can help prevent or reduce these effects. In fact, we have now developed a range of antihistamines that can also help with other symptoms or conditions such as insomnia and nausea. Read on as we get into more detail about commonly asked questions like “antihistamines how do they work?” or “how do allergy tablets work?”.

What Are Antihistamines?

Antihistamines are a type of medication often used to manage symptoms such as hives, runny nose, itchy eyes (i.e allergic symptoms) but can also be used to manage nausea, motion sickness, and even insomnia. Versatile right? In addition to its many uses, antihistamines can come in many different forms from nasal sprays to tablets. Many antihistamines can be bought over the counter at supermarkets or pharmacies or online providers such as Meds for Less. Let’s get into the question of “how do antihistamine work?”

How Antihistamines Block Histamine Receptors in the Body

So how do antihistamines work? As the name suggests, the antihistamines work by being “anti – histamines” as in blocking histamine receptors. There are two main ways the medication does this blocking: 1) by binding to the site where histamine is meant to bind or 2) or making the receptor stay in its inactive site. By blocking the histamine receptors, this stops the activation of receptors which cause the blood vessels to dilate and become more permeable/leaky; this then causes the allergic reaction (e.g sneezing, itchiness). Additionally, as there are many kinds of antihistamines for different purposes, it might be helpful to go through the types of receptors that are targeted and what effects they produce:

H1 Receptor Antagonists

The H1 receptors are found around the body (e.g in smooth muscle, in the airways). Normally, histamine binds to the H1 receptor and brings about the inflammatory response. Thus, the H1 receptor antagonist type of antihistamine blocks the receptor from being activated and then stops the allergic response.

H2 and Other Histamine Receptor Types

The H2 receptors are found in the stomach where they produce stomach acid. The H2 receptor antagonist antihistamines thus block these receptors and reduce stomach acid secretion. There are four types of histamine receptors but the main ones targeted by antihistamine medications are H1 and H2.

How Long Do Antihistamines Take to Work?

So how long do antihistamines take to work? They work quite quickly, you will probably start to notice relief from symptoms within half an hour to 2 hours. The medium of administration (e.g tablet vs spray) also can affect how quickly the medication takes effect. Eye drops and nasal spray (i.e topical) antihistamines can help relieve allergic symptoms at the target site more quickly than oral tablets. So if you suffer from hay fever for example, it is helpful to take the antihistamine regularly before you go out rather than wait for the symptoms to come on.

How Long Does the Effect of Antihistamines Last?

How long the antihistamine’s effect lasts depends on what type of antihistamine you are taking. For example, antihistamines such as Chlorpheniramine that cause drowsiness will last longer (e.g around 4 to 6 hours). On the other hand, one dose of the non-sedating antihistamines such as Loratadine or Cetirizine typically lasts around 12 to 24 hours.

Factors Affecting How Quickly Antihistamines Work

We already covered “how fast do antihistamines work?” but what factors can affect this? Let’s get into some key things to keep in mind when you are taking antihistamines:

Formulation and Dosage

The formulation (e.g nasal spray vs oral tablet) can be a factor in how quickly antihistamines work. For example, topical (nasal spray, eye drops) antihistamines may act quicker on the target area compared to oral or liquid form antihistamines that you ingest. Regardless, overall antihistamines work quite quickly to relieve allergic symptoms. Another key medication factor is the dosage, make sure to read the specific instructions on the medication package to ensure you are taking the right dose for your age as it may affect your response to the antihistamine. Please consult a pharmacist or your healthcare provider if you are unsure about what dose to take or about contraindications or interactions with other medications for chronic conditions that you may be taking.

Whether Taken With or Without Food

The effect of food on the absorption of antihistamines depends on which antihistamine you are taking. For example, it does not matter if you take Cetirizine on an empty stomach or after food, but Fexodenadine has to be taken on an empty stomach as food can reduce its effectiveness. Check with your healthcare provider if you are unsure about how to take the specific antihistamine you are using.

Individual Sensitivity

Much like a lot of medications, individual differences in the response to antihistamines can mean symptom relief happens very quickly for some people but more slowly for others. This could be due to a range of reasons from pre-existing medical conditions, individual sensitivity to histamine, or individual differences in metabolism. Sometimes you might find that you have to try a few different antihistamines before you can find out which one works best for you.

When to Use Antihistamines – Common Conditions and Symptoms

Now that we’ve gone through the main question of “antihistamine how do they work?”, let’s cover when we use them. Antihistamines can be used to manage a large range of conditions and symptoms, let’s go through a few common ones:

• Conjunctivitis
• If you have hay fever, antihistamines can help relieve the uncomfortable runny nose, itchy eyes, and sneezing. You can take antihistamines regularly during seasons when pollen is high.
• Hives
• Insect bite reactions (e.g if you notice itchiness, swelling, or redness)
• Short-term difficulties sleeping. There are some sedating antihistamines that can help with this.
• Motion sickness and nausea

If you experience any of these symptoms or conditions, you may benefit from antihistamines. Please consult your healthcare provider who can recommend a suitable antihistamine or other treatment for you.

Side Effects and Safety Considerations

As with all medications, although antihistamines are very safe and widely used, there are some side effects and safety considerations that you should be aware of. It is useful to go through this by covering sedating and non-sedating antihistamines separately.

Some more common side effects include:

If you notice any unusual side effects or an allergic reaction (swelling, feeling like you cannot breathe, fainting etc.) after taking an antihistamine please seek emergency medical attention. This is a rare complication but important to be aware of regardless.

Some safety considerations to help you take antihistamines safely include:

• Not drinking alcohol when taking sedating antihistamines. There is a risk that you will become even drowsier.
• If you take any other medication regularly or you have a pre-existing medical condition, please consult your healthcare provider for advice on what antihistamines are best suited and safe for you.
• If you are trying to conceive, you are pregnant, or you are breastfeeding, Loratadine and Cetirizine are generally considered safe to use. It is still best to check with your healthcare provider depending on what you need an antihistamine for.
• For children, make sure to follow the specific instructions on the medication package to give the appropriate dose. Consult your healthcare provider for advice on which specific type of antihistamine to use.

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