Antihistamines Are One Piece of the Puzzle... Not the Whole Picture....

If you've seen my recent reel about antihistamines and hot weather, you'll know it gained quite a lot of attention!

So I wanted to expand on my recent reel because the conversation deserves a little more context. The important thing to understand is that not all antihistamines behave in the same way.

Certain antihistamines possess anticholinergic properties, meaning they block the action of the neurotransmitter acetylcholine. Acetylcholine is responsible for activating your eccrine sweat glands, which produce sweat across most of your body. Sweating isn't just something that happens when you're hot—it's your body's primary cooling mechanism. As sweat evaporates from your skin, it helps regulate your core body temperature. If that process is impaired, particularly during periods of extreme heat, your risk of heat exhaustion and heat stroke increases. This is why medications with anticholinergic properties are frequently highlighted in public health advice during heatwaves.

The extent of that risk depends very much on which antihistamine you're taking.

First-generation (older, sedating) antihistamines are the main culprits because they have much stronger anticholinergic effects. Examples include diphenhydramine (Benadryl), chlorphenamine (Piriton), promethazine (Phenergan) and doxylamine.

These medications are much more likely to reduce sweating, as well as cause drowsiness, dry mouth and blurred vision. If you're taking one of these during a heatwave or whilst on holiday somewhere hot, it's sensible to be extra mindful of staying hydrated, keeping cool and recognising the early signs of heat exhaustion.

By comparison, second-generation (newer, non-sedating) antihistamines such as cetirizine (Zyrtec), loratadine (Clarityn), fexofenadine (Telfast/Allegra), desloratadine and levocetirizine were designed to be much more selective. They have minimal anticholinergic activity and are therefore far less likely to interfere with sweating or the body's normal temperature regulation.

It's also important to remember that antihistamines may not be the only medications influencing your body's ability to regulate heat. Certain antidepressants, bladder medications, antipsychotics, anti-sickness medicines and even some cold and flu remedies can also have anticholinergic effects. When several of these are taken together, their effects can become additive, increasing the overall anticholinergic burden. And don't forget that every medication you take has to be processed by the liver. And an overburdened liver can be one of the contributing factors to MCAS.

None of this is intended to make you fearful of taking medication. It's intended to do what I hope all of my content does: empower you with knowledge.

Knowledge allows you to make informed decisions that are right for your body, your health and your circumstances.

There's a big ol' chapter in The Histamine Handbook dedicated to antihistamines because I genuinely believe they have an important place. They can be incredibly helpful, in acute phases and for many people they're an essential part of managing symptoms. I've taken them myself, and I know first-hand the difference they can make.

When my own health developed into full-blown Mast Cell Activation Syndrome (MCAS), antihistamines helped steady the ship when I genuinely thought something catastrophic was happening. (I mean it was, but just not the brain tumour I thought I had because I could not stand without falling over!) They didn't cure me, but they reduced the intensity of my symptoms enough for me to breathe again, calm my nervous system and give my body a chance to recover from the immediate crisis.

Sometimes that's exactly what medication is there to do.

There was a time when I was taking Benadryl, Famotidine, Fexofenadine, Sodium Cromoglicate and Ketotifen every single day because that's what my body needed at that stage of my journey. Today, I rarely take antihistamines on a daily basis because I've spent years working on the underlying factors driving my mast cell activation. But that doesn't mean I've sworn them off forever. Just this week, after being exposed to bleach, rancid essential oils and extreme heat / stress all in one day, I recognised my mast cells beginning to flare. Rather than waiting until I was sofa-bound with dizziness, nausea and overwhelming fatigue, I chose (alongside a bucket of natural remedies and protocols) to use an antihistamine acutely to calm things back down.

That's the difference understanding your body can make.

For me, medication isn't about "always" or "never."

It's about knowing when, why and how to use it as one tool within a much bigger toolkit.

Where I think we need to shift the conversation is in understanding what antihistamines can do... and what they can't.

Antihistamines work by blocking histamine receptors, reducing the effects histamine has on the body. They can make an enormous difference to quality of life. What they don't do, however, is address the underlying biological processes driving excessive histamine release or mast cell activation.

That distinction is incredibly important. I often use the analogy of a smoke alarm. Imagine the alarm starts blaring in the middle of the night. You could remove the batteries and stop the noise, but the fire would still be burning. The alarm wasn't the problem—it was alerting you that something else needed your attention.

For me, antihistamines are a little like that. They help quieten the alarm, but they don't necessarily deal with what's causing the alarm to sound. That's why, whenever somebody tells me antihistamines have helped them, my next question isn't,

"Great... job done." It's...

Why did your body need them in the first place?

Is your gut involved?

Are hormones playing a role?

Has mould exposure tipped the balance?

Could chronic stress, poor sleep, nutrient deficiencies or environmental chemicals be contributing?

Because these are the questions that move us beyond simply managing symptoms and towards understanding what's actually driving them.

This is where I think conventional medicine and functional medicine can work beautifully together, rather than being seen as opposing approaches. Conventional medicine often asks, "What medication can we use to reduce this symptom?" 

Functional medicine asks a different question: "Why is this symptom happening in the first place?" Neither question is wrong. Medication can help you feel better today. Root-cause work aims to improve your health tomorrow. One helps calm the waves. The other tries to understand what's causing the storm.

That's exactly why I wrote The Histamine Handbook. I didn't want to produce another guide that simply listed foods to avoid. I wanted to explain the why. Why histamine accumulates. Why hormones, gut health, mould, nervous system regulation, nutrient status and environmental exposures all matter.

Because when you understand the why, you stop feeling frightened by your body. You start understanding it. And that's where real progress begins.

If you're travelling somewhere hot this summer, or simply spending more time outdoors, I'd encourage you to pocket this knowledge. Maybe watch my short reel on antihistamines and sweating. It's information that could help you make safer, more informed decisions if you're taking these medications.

And if you're ready to understand the bigger picture, that's exactly what The Histamine Handbook was written to help you do.....

Big love

Jo

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