What caffeine does to your body after minutes, hours and a lifetime

For many of us, a cup of tea is the first thing on our mind when we wake, a mid-morning coffee essential to keep us going and a can of cola, a reward we rely on when we reach the end of a long day. Of course, we like the taste, but the caffeine keeps us hooked, influencing everything from our blood pressure to our brain function. So exactly how quickly do the effects of this drug take hold – and how can we maximise the benefits and reduce unwanted side effects? Within four minutes you might need the loo No, you’re not imagining it – that coffee can make you need the toilet, because “it stimulates colonic motility, which is why many people feel the urge to go shortly after a cup,” says neuroscientist and neuro-gastroenterologist Dr Madusha Peiris. This seems to be because of its other compounds as well as the caffeine – a study in the European Journal of Gastroenterology & Hepatology found caffeinated coffee makes the colon 60 per cent more active than water and 23 per cent more active than decaf. An old but intriguing study in the journal Gut found the desire to defecate came on within four minutes of ingestion and lasted at least 30 minutes. So time your latte with your morning break. Shorts Ten minutes until your mind is altered Caffeine enters your bloodstream (mainly via the small intestine lining) quickly and “you’ll typically start to feel its effects within around 10 minutes”, says Dr Graeme Kirkwood, cardiologist at healthcare provider LivingCare Group. It blocks a brain chemical called adenosine that builds up during the day and makes you feel tired. “It can improve alertness, that perception of being able to concentrate,” says registered dietitian Kate Hilton, who works with the Institute for Scientific Information on Coffee (ISIC). Most studies suggest you should consume caffeine between 30 and 60 minutes to reap the benefits when doing something cognitively demanding. But caffeine content can vary widely. A recent Which? study found a medium Starbucks cappuccino had less than a third of the caffeine of a Costa equivalent – which had four times the caffeine as a can of Red Bull. Your blood pressure may rise in 30 minutes Caffeine causes your blood vessels to constrict and a “measurable” rise in blood pressure can occur within half an hour, says Kirkwood. “At a cellular level, it also influences calcium handling in heart muscles, and can make the heart contract a little more strongly. You might feel your heart beating faster or more forcefully, but that doesn’t necessarily mean anything is wrong.” Those used to caffeine can build a tolerance, but for people unaccustomed to it, 400mg (around four mugs of coffee) is enough to keep blood pressure raised for three hours after ingestion, according to a study in the journal Pharmacotherapy. Most health authorities recommend healthy adults stick to 400mg caffeine a day, while the NHS recommends pregnant women limit themselves to 200mg a day. Coffee can make you need the toilet, says neuroscientist and neuro-gastroenterologist Dr Madusha Peiris After 39 minutes, levels in your saliva could peak A study in Pharmacology, Biochemistry and Behaviour found that after drinking 400mg caffeine in a cola solution, participants’ saliva levels peaked after 39 minutes, and after 400mg in coffee, 42 minutes. Yet some levels peak as quickly as 15 minutes, and others not for 120 minutes. The effects may improve your muscles as well as your mind, adds Hilton, because caffeine prior to a workout can make it feel easier. A 2020 trial in the journal Frontiers in Nutrition found that those who took a caffeine solution 60 minutes before exercise showed better improvement in strength and jumping abilities than those who took it 30 minutes or two hours before. …Unless you’re drinking it with dinner, in which case it might be three hours If you’ve had a caffeinated drink with lunch or dinner, you might not feel all the effects for three hours, according to an Australian study in the Journal of Science and Medicine in Sport, which found that participants who consumed caffeine with a carbohydrate-rich meal saw levels peak after two to three hours, compared with 60 minutes for those who ingested it in a fasted state. A substantial meal slows caffeine’s transit to the small intestine, explains Peiris. “The effect therefore feels gentler and more sustained rather than a sharp lift.” Still, “this does not mean we become less sensitive to caffeine, just that the caffeine releases over a longer window”. Over eight hours until it’s gone from your body Caffeine’s “half-life” – the amount of time it takes for the caffeine, metabolised by the CYP1A2 enzyme in your liver, to fall by half, is typically four to six hours. Variables such as whether you’re pregnant or have impaired liver function can extend it, but smoking – which speeds up caffeine metabolism – shortens it. Hilton says the time it takes for caffeine to leave your body altogether could be eight hours, although for some “it can take 10 or 12 hours” and there can still be plenty of caffeine circulating in your body even if you don’t feel the effects. “Generally, six hours before bedtime is the place you need to stop drinking it in order to not have an effect on your sleep.” Withdrawal symptoms last up to nine days Consume caffeine regularly and the brain adapts by increasing the sensitivity of adenosine receptors. If you cut it out, adenosine will therefore act more strongly, inducing the fatigue associated with withdrawal. Headaches happen when blood vessels in the brain constricted by caffeine experience “rebound dilatation” that is “possibly because of increased sensitivity to circulating adenosine”, according to a study in the journal Psychopharmacology. A review in the same journal found that the onset of withdrawal symptoms usually started between 12 and 24 hours after quitting, and lasted up to nine days. American doctors Franklyn Rocha Cabrero and Richard J. Hamilton recommend reducing caffeine gradually, remaining hydrated and taking ibuprofen to manage withdrawal symptoms in their 2025 clinical review on caffeine withdrawal on the medical database StatPearls. Registered dietitian Kate Hilton, who works with the Institute for Scientific Information on Coffee, says the time it takes for caffeine to leave your body altogether could be eight hours Three years of consumption might reduce your body fat Up to seven cups of coffee a week was found to be associated with a reduction in fat in a 2023 study in the journal Clinical Nutrition. It tracked 1,483 participants over three years, with study authors concluding that “moderate consumption of caffeinated coffee may be part of a weight management strategy”. Peiris concedes that “caffeine raises resting energy expenditure by a small amount through stimulating the sympathetic nervous system, and it increases release of fatty acids from fat stores”. But while observational studies tend to find that coffee drinkers weigh slightly less over time, “those studies cannot separate the coffee from the habits of the people who drink it”. And, she adds, “I would not recommend coffee as a weight-loss strategy because the effect is too small to be a meaningful intervention on its own, and increasing intake for that purpose can impact sleep and anxiety.” Drink it throughout your life and you might lower your risk of dying early A 2024 review of nearly two million people in the journal Epidemiology and Health found that those who drank around two cups of tea a day had the biggest reduction in all-cause mortality, while a review in the BMJ linked three to four cups of coffee a day with a 17 per cent lower risk of all-cause mortality. However, decaffeinated tea and coffee are also linked to lower mortality, and there is no good evidence linking cola to a longer lifespan. Hilton says antioxidants in caffeinated drinks are thought to be responsible for reduced frailty in later life, rather than caffeine, but “more research is needed”.