5 signs you could be intolerant to caffeine
In 2026, there’s almost nothing we don’t know about our bodies. From how we sleep to how the food we eat interacts with our microbiome to exactly what our blood sugar gets up to throughout the day, rare is the metric that remains a mystery. And I recently decided to take things a step further: I completed the 10X Health System genetic test, to learn what secrets were lurking in my genes and how I could positively harness the information for the benefit of my health.
Besides having a slightly dubious detoxification system, the main takeaway from my results was my body’s genetic inability to process caffeine. I’d long suspected I had a systemic intolerance to coffee, matcha and caffeinated tea, largely because of how off-kilter it makes me feel.
You may not want to know whether or not your body is particularly adept at processing caffeine and I respect that – so long as you’re not pounding a dozen espressos a day, you should be fine ( current guidelines suggest adults should not exceed 400mg of caffeine a day, which is the equivalent of four to five cups of coffee or matcha). If you are interested, though, read on.
How your genes determine your caffeine intolerance
Prepare yourself for some unpronounceable genetic terms. “The CYP1A2 enzyme gene in the liver is responsible for breaking down roughly 95 percent of all caffeine consumed,” Dr Pascale Ricci, head of precision nutrition at Reviv Global, a brand that works alongside 10X Health, tells me. “The rs762551 SNP (single-nucleotide polymorphism) determines how fast or slow it occurs.”
Right, let’s break that down: rs762551 is a variation of the CYP1A2 enzyme gene. It’s what determines how quickly or slowly your body can break down caffeine. Committing either term to memory is not important, but understanding the role they play in your caffeine tolerance is. A genetic test, like the one I did, produces one of three results: you’re either an AA, AC or CC.
Approximately 41 percent of people are AA or fast metabolisers of caffeine, meaning their CYP1A2 enzyme works efficiently. “Caffeine is cleared relatively quickly, with a typical half-life of around two and a half to four and a half hours,” Ricci explains. If you know someone who can have a fresh cup of coffee at 6pm and still sleep soundly, chances are they’re an AA.
Join me in the middling club, along with around 43 percent of the population! An AC gene variant suggests that one of your parents gave you a functioning gene and one gave you a defective one. This makes us okay at metabolising caffeine: not brilliant, not terrible.
Finally, 16 percent of the population are estimated to be slow caffeine metabolisers, with two copies of the slower gene variant. “This means caffeine lingers in the bloodstream for significantly longer and leads to higher plasma caffeine levels after a cup of coffee,” when compared to someone with an AA variant, Dr Ricci explains.
“In simple terms, if two people drink the same latte, the slow metaboliser (CC) will have caffeine circulating in their body for much longer than the fast metaboliser (AA), meaning the effects (both the good and the bad) last longer and feel stronger.”
How to tell which type of caffeine metaboliser you are
Short of a genetic test, Dr Ricci has a few pointers of things to look out for when thinking about how well you metabolise caffeine.
Difficulty falling asleep and poor sleep quality, even when keeping caffeine consumption to the morning, can indicate slow caffeine metabolisation.
“Anxiety, jitteriness or nervousness, particularly when consuming over 200mg of caffeine,” is another sign slow metabolisers may notice, she says. A prolonged “wired” feeling is another telltale sign.
A pounding heartbeat or palpitations can be an indication of a caffeine reaction, though they can also be a sign of other, more worrying health concerns. Contact the emergency services or your GP if a new symptom arises, persists or becomes concerning.
An upset stomach, acid reflux or diarrhoea can all be signs of caffeine intolerance, though they may also have other causes.
For a definite answer, a genetic test is the way to go. It proved my suspicions: thanks to one of my parents, an AC gene variation meant my body struggled to break down caffeine and, according to my report, is best avoided.
How to enjoy caffeine regardless of results
Dr Ricci suggests curbing your daily caffeine intake to less than 200mg a day, which is the equivalent of one cup of coffee. A double-shot of coffee contains anywhere between 120mg and 160mg of caffeine, while a single shot would sit in the 60mg to 80mg range.
“Front load caffeine by drinking it in the morning,” Dr Ricci recommends. “Caffeine lingers for longer in slow metabolisers and consuming it later in the day is more likely to disrupt sleep.” Find a cut-off point that works for your lifestyle and make it a hard stop. That could be after your first cup or go by the clock and avoid it altogether after midday.
If you want to drastically cut down but can’t abandon the habit entirely, green tea and half-caffeinated coffee are good options. Green tea contains around 30mg to 50mg of caffeine per cup, which, when swapping from a double-shot habit, is a marked reduction.
A good rule of thumb is to make every follow-up drink (whatever you reach for after you’ve had your one or two cups of coffee) a decaffeinated one.
Dr Ricci warns that some common medications (“antibiotics, certain cardiovascular drugs, oral contraceptives and some anti-depressants”) slow down the CYP1A2 enzyme gene activity, which can compound the already lingering effects of caffeine. “You may need to reduce the amount of caffeine you drink further when starting new medication,” she cautions. The best approach is to speak with your doctor when starting any new medication or supplement.
“Tapering off caffeine, rather than quitting abruptly, tends to reduce overall consumption,” Dr Ricci finishes. “It also helps to minimise withdrawal symptoms like headaches and fatigue.”