I've lived in South Korea, Vietnam, Thailand, Cambodia, spent chunks of time across Southeast Asia, and a year in Melbourne. In every single one of those places: no hayfever. Then I moved to the UK and within one summer I was waking up with eyes nearly sealed shut, going through a box of tissues before 9am, and genuinely questioning whether a run in June was worth the suffering. What made it stranger was that Melbourne had already given me a preview. A few weeks in Australia and suddenly I had symptoms I'd never experienced in my life, despite spending years in countries where I'd never had so much as a sniffle. I assumed it would pass. It didn't, and when I arrived in England it got considerably worse. If you've had a similar experience, or if you've been running through British summers wondering why your easy pace feels harder in May than in November, there's a specific reason for it, and the standard advice most people get about managing hayfever doesn't cover half of what's actually going on.
I lived in twelve countries and got hayfever in two of them
The UK and Australia have something in common that most of Southeast and East Asia doesn't: a grass pollen season that is both intense and precisely timed to coincide with the months most runners care about most. Ryegrass and timothy grass are the main culprits in Britain, producing pollen in large volumes from May through August. Australia has similar species plus some aggressive native plants that go to work in spring. Much of Southeast Asia has different grass species, higher year-round humidity, and rainfall patterns that keep airborne pollen counts low for most of the year. So if you spent time in those parts of the world and never had hayfever, you weren't imagining a difference. The pollen was genuinely different, and your immune system had simply never encountered these particular proteins before.
When I settled in the UK and started running regularly, I got the full seasonal sequence: tree pollen in spring, grass pollen through summer, mould spores in autumn. Three separate windows of something trying to make training miserable.
What hayfever actually does to your running
Most people treat hayfever as an annoyance rather than a performance variable. It's worth being precise about what it's actually doing to your physiology during a run, because it goes well beyond sneezing at inopportune moments.
When your immune system reacts to pollen, it triggers the release of histamine, which causes the airways in your nose and throat to swell and produce excess mucus. This forces you into mouth breathing earlier and at lower intensities than usual. Mouth breathing bypasses the nose's natural filtering and humidifying function, meaning drier, less filtered air reaching your lungs, a higher rate of water loss, and a noticeable increase in perceived effort at any given pace. A 2016 study in the International Journal of Sports Medicine found that runners with allergic rhinitis showed measurably reduced exercise capacity during high-pollen periods compared to other times of year, with nasal congestion identified as the primary driver.
Beyond the airway issue, your body is running a low-grade inflammatory response throughout the entire hayfever season. That's additional immune system activity sitting on top of the normal stress of training, which means slower recovery between sessions and a reduced capacity to adapt. If your easy runs feel harder in June than they did in March despite similar mileage, this is likely part of the reason.
Do antihistamines affect your training?
This is the question I spent a long time looking for a straight answer to, and the honest response is: it depends on which one you're taking and when.
First-generation antihistamines, the older ones like chlorphenamine (Piriton) and promethazine, cross the blood-brain barrier and cause sedation. Research published in the British Journal of Clinical Pharmacology found that diphenhydramine, a common first-generation antihistamine used in many sleep and cold remedies, significantly reduced psychomotor performance, reaction time, and alertness. If you've ever taken an older antihistamine and gone for a run feeling strangely flat and heavy-legged, that's the mechanism: your central nervous system was partially suppressed, and your perceived effort at any given pace was higher than it should have been.
Second-generation antihistamines, including cetirizine (Zirtek), loratadine (Clarityn), and fexofenadine (Telfast), have a much weaker effect on the brain and are generally considered fine for athletes. Fexofenadine has the least central nervous system activity of the three. A review published in Allergy found no meaningful effect on exercise performance from second-generation antihistamines at standard doses, with the caveat that high doses of cetirizine can still cause mild sedation in some people.
There's one more thing worth knowing. Histamine plays a role in vasodilation during exercise, helping blood flow to working muscles. A 2010 paper in the Journal of Applied Physiology by Romero and colleagues suggested that blocking histamine receptors completely during prolonged exercise may slightly reduce skeletal muscle blood flow. The effect appears small, and the study was conducted under controlled conditions rather than real-world running, but it does mean that taking antihistamines before every run indefinitely isn't entirely without trade-offs. For most recreational runners the benefit of clearing your airways easily outweighs this. But it's a reason to use them sensibly rather than treating them as completely neutral.
The practical takeaway: use a second-generation antihistamine, take it the night before rather than immediately pre-run, and keep first-generation antihistamines well away from any day you plan to train.
The recovery thief nobody mentions
The hayfever impact that most affected my training wasn't the runs themselves. It was the nights before them.
Nasal congestion at night forces mouth breathing during sleep, which fragments sleep architecture in well-documented ways. Nasal obstruction increases the likelihood of sleep-disordered breathing, reduces time spent in slow-wave deep sleep, and raises overnight cortisol levels. Slow-wave sleep is when the majority of physical recovery and tissue repair happens. Disrupt it consistently across six weeks of pollen season and your training adaptation suffers, even if every individual session looks fine on paper.
I noticed this pattern long before I understood why: harder to get out of bed in June than in October, legs heavier on Tuesday than they should be after a gentle Monday, minor niggles that would normally clear in two days taking five. Once I started taking my antihistamine at night rather than in the morning, sleep quality improved noticeably, and the knock-on effect through training weeks was real. This is probably the single most useful adjustment I've made for managing hayfever as a runner, and it's almost never mentioned in the standard advice. Most people take their tablet with breakfast and wonder why they're still congested at midnight.
Nasal saline rinses before bed also made a genuine difference. Unglamorous but effective: they clear residual pollen from the nasal passages and reduce overnight congestion without any of the rebound effects that nasal decongestant sprays cause if you use them long-term.
What actually helps
Based on what's worked for me and what the research supports, here's what I'd tell another runner trying to keep training through hayfever season.
- Switch to a second-generation antihistamine if you haven't already. Cetirizine, loratadine, or fexofenadine. All available over the counter, all significantly better for training than older options. If one doesn't suit you, try another — responses vary between individuals.
- Take it at night, not in the morning. It covers the high-pollen evening period, protects your sleep quality, and the active period extends into the following morning when pollen counts are still building. Most people do the opposite.
- Run earlier in the day. Grass pollen peaks between late morning and early afternoon, then rises again in the early evening. A 6am run in June is often considerably more comfortable than one at 11am. The Met Office pollen forecast is worth checking the night before.
- Nasal saline rinse before bed. About £4 from any pharmacy. Clears pollen from the nasal passages, reduces overnight congestion, and has no side effects with daily use.
- Skip the Vaseline-around-the-nostrils trick. You'll see this recommended frequently. The idea is that it traps pollen before it enters the nose. The evidence is thin and it tends to make things worse for anyone who's also dealing with skin sensitivity. The practical options above are more reliable.
- Work on nasal breathing during easy runs. Uncomfortable at first, but training yourself to breathe through your nose at lower intensities keeps air filtered and reduces the volume of pollen reaching your lower airways. It also means mouth breathing feels less desperate when you actually need it at higher efforts. Start with the warm-up and cool-down and build from there.
- Adjust expectations for high-pollen weeks. A measurably harder easy run in June isn't a fitness problem. Your body is dealing with an additional inflammatory load and reduced sleep quality. Treating those weeks as reduced-capacity blocks rather than pushing through them at normal effort tends to produce better results over the full season.
Getting out the door anyway
Hayfever season in the UK runs roughly from March to September depending on which pollens affect you most, so it overlaps with almost every race most runners are building toward. Waiting it out isn't a training strategy. Managing the variables you can control, which antihistamine you're taking, when you're taking it, when you run, and how seriously you're treating sleep, gives you most of your summer back.
I still get hayfever. Some mornings in June I still look like I've been crying into a hedge. But the bad weeks are fewer than they used to be, and since I started treating sleep disruption as the main problem rather than the daytime symptoms, it doesn't derail training the way it once did. If you take one thing from this: switch to a second-generation antihistamine and take it at night. Everything else is a bonus.
