How much coffee can a healthy adult drink?

Question

How much coffee can a healthy adult drink before it starts to affect them in a bad way?

For example, some friends of mine get headaches if they drink more than 2 cups in a single day.

Answer 1

There are many factors in determining how much coffee one can drink in a day. Caffeine metabolism¹ is a big one. People who metabolize caffeine slow, probably shouldn't have more than 100mg of caffeine (about a cup) a day. Fast metabolizers of caffeine can usually have between 400-500mg (about 5 cups) a day.² For the general population, 400mg (about 4 cups) should be the limit. Going over this may cause several side effects such as insomnia, restlessness, headaches, and upset stomach.³ It usually isn't very hard to know your limit of drinking coffee. If coffee has a strong effect on you right away, drink less coffee, or consider drinking tea. If coffee has a weak effect on you, then you can drink more. Most people will fall somewhere in between.

---

^{[1] Why do I feel shaky after only a small amount of caffeine?}

^{[2] Caffeine Consumption}

^{[3] Caffeine: How much is too much?}

Answer 2

It really depends on the coffee which its caffeine content (its main psychoactive substance) can vary by size, bean origin, roast method and other factors.

Up to 400 milligrams (mg) of caffeine a day appears to be safe for most healthy adults. That's roughly the amount of caffeine in four cups of brewed coffee, 10 cans of cola or two "energy shot" drinks. Although caffeine use may be safe for adults, it's not a good idea for children. See: Caffeine: How much is too much? at Mayo Clinic

Higher consumption of 1000–1500 mg per day is associated with a condition known as caffeinism.

---

Caffeinated beverages like coffee and soft drinks give short bursts of energy, but can actually cause fluid loss. Caffeine has been shown to temporarily raise blood pressure, and reduces blood flow to inactive limbs.

Like many drugs, caffeine is chemically addictive and recent publication of Diagnostic and Statistical Manual of Mental Disorders (DSM), caffeine withdrawal was finally included as a mental disorder for the first time.

Soon after you drink coffee (containing caffeine), it’s absorbed through the small intestine and dissolved into the bloodstream and it’s able to penetrate the blood-brain barrier and enter the brain.

Regular ingestion of the drug (i.e. coffee/tea, soda or energy drinks) alters your brain’s chemistry and physical characteristics actually change over time, leading to fatigue, headaches and nausea if you try to quit. However, compared to many drug addictions, the effects are relatively short-term.

Few quotes from Daily Mail:

Large amounts of caffeine can cause heart palpitations, fits and even death, as well as raising the risk of Type 2 diabetes.

Heavy consumption has also been linked to a greater risk of depression, addiction and alcohol dependency.

Energy drinks could be more likely to cause a caffeine overdose because they can be drunk quickly, unlike hot drinks like tea or coffee, the Energy Drink Consumption in Europe study said.

Several deaths worldwide have been linked to excessive consumption of energy drinks, although scientists say more research is needed to prove a link.

Recommended maximum caffeine intake is about 400mg per day – equivalent to around five cups of filter coffee.

'Reproductive-aged women (⩽ 300 mg caffeine per day) and children are ‘at risk’ subgroups who may require specific advice on moderating their caffeine intake', study said.

On Live Science we can read:

Coffee drinking could lead to a mental disorder. If you experience five or more symptoms, such as red face, nervousness and restlessness, during or right after your cup of Joe, you may be diagnosed with coffee intoxication.

According to a new edition of the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (DSM-5), excessive caffeine intake can lead to a condition known as “caffeine intoxication,”.

In other words, caffeine withdrawal is now a recognized disorder, and is listed in the DSM-5.

“Caffeine is a drug, a mild stimulant, which is used by almost everybody on a daily basis,” said Dr. Charles O’Brien, who chairs the Substance-Related Disorder Work Group for DSM-5 (via New York Post). “But it does have a letdown afterwards. If you drink a lot of coffee, at least two or three [236 ml] cups at a time, there will be a rebound or withdrawal effect.”

Here are top 10 caffeine withdrawal symptoms:

1. Headache
2. Sleepiness
3. Irritability
4. Lethargy
5. Constipation
6. Depression
7. Muscle Pain/Stiffness
8. Lack of Concentration
9. Flu-like symptoms
10. Insomnia

In rare cases when consumed at high enough doses, caffeine can kill.

---

Related:

• Health effects of caffeine at Wikipedia
• Effect of caffeine on memory at Wikipedia
• (article) Sales of sugar and caffeine laden energy drinks should be restricted as they 'pose a danger' to children at Daily Mail
• What's the minimum recommended age for drinking a coffee?
• This Is How Your Brain Becomes Addicted to Caffeine at Smithsonian
• What Caffeine Actually Does to Your Brain at Life Hacker
• (study) Effects of Caffeine on Human Health, P. Nawrot, S. Jordan, J. Eastwood, J. Rotstein, A. Hugenholtz and M. Feeley, Food Additives and Contaminants, 2003, Vol. 20, No. 1, pg. 1-30.
• (study) Actions of Caffeine in the Brain with Special Reference to Factors That Contribute to Its Widespread Use
• Caffeine Withdrawal Is Now a Mental Disorder at Time
• (book) Buzz: The Science and Lore of Alcohol and Caffeine by Stephen Braun

Answer 3

While this is not a complete answer, I'm pointing out here that coffee intake is adverse to iron absorption. Caffeine does play a role here, although I don't know to what extent other substances in coffee do. I have asked a separate question here.

For this reason, it is recommended not to drink coffee together with, or directly after iron-rich meals (say, a breakfast with oatmeal and fruits) since the intake and the benefits of the iron would be inhibited. This applies especially to people who have low haemoglobin levels, but also to healthy adults who want to keep their iron levels up. Pregnant women should be especially careful and best avoid coffee completely since coffee consumption is linked to iron deficiency anemia in their infants.

Answer 4

I recently read a fantastic article in the first issue of National Geographic Science. To sum it up for you, coffee is actually great for you. 3 cups a day can decrease mortality by 12%. Right now I do not have the magazine on me but here is a shorter version. My only concern is caffeine.