Side effects of caffeine


  • Caffeine can be bad for you, if you are a slow caffeine metabolizer, are allergic to caffeine, have caffeine overdose, or reached the caffeinism stage.
  • High caffeine intake may have laxative effects in some people.
  • Caffeine can worsen reflux symptoms in people that suffer from GERD.
  • Caffeine intake by obese individuals is associated with indigestion.
  • Hypertensive or highly caffeine sensitive people may have blood pressure increase after drinking caffeinated drinks.
  • Caffeine can increase the risk of heart attack in slow metabolizers.
  • Caffeine may cause dependent anxiety and nervousness.
  • Caffeine can disrupt your energy levels throughout the day and affect your sleep.
  • Large doses of caffeine may increase the risk of infertility.
  • Pregnant women who are slow caffeine metabolizers should avoid caffeine intake to prevent miscarriages.
  • Women in the breast cancer risk group should avoid caffeine to reduce the risk of fibrocystic breast disease.
  • Postmenopausal women may experience more hot flashes and night sweats from caffeine usage.
  • Caffeine supplements may induce new seizure occurrences in people with seizure disorders.
  • Caffeine consumption can decrease glucose uptake and insulin sensitivity.


Side effects of caffeine – who is at risk?

Caffeine is a drug that can be safely consumed by most adults in the amounts of up to 400mg per day (about 4 cups of espresso coffee). For some people caffeine can be beneficial, but not everyone can enjoy a few cups of coffee without experiencing unwanted side effects of caffeine. The variation in the effects of caffeine are mostly determined by our genes.

Negative health consequences are mostly associated with:

  • caffeine overdose,
  • caffeine withdrawal symptoms,
  • caffeinism,
  • high sensitivity to caffeine, and
  • allergic reaction. 

Side effects of caffeine can manifest as either:

  • Temporary effects from caffeine overdose, withdrawal symptoms or allergic reaction, or
  • Long-term health complications that result from regular caffeine consumption.

To determine whether you can have caffeinated drinks or not, it is necessary to consider both the beneficial and side effects of caffeine and determine which may apply to you.

It is especially worth learning about how caffeine can affect you in the long-term if caffeinated drinks are part of your daily diet.

This article talks about the long-term effects of caffeine usage.

Caffeine overdose 

You can experience a caffeine overdose, regardless of whether you are sensitive to caffeine or not. The severity of the overdose depends on a combination of the caffeine dose and your level of caffeine sensitivity.

For instance, you can be extremely sensitive to caffeine and a few sips can be enough to cause the onset of symptoms.

Caffeine withdrawal symptoms

You start experiencing caffeine withdrawal symptoms between 12 and 24 hours after discontinuation of regular caffeine intake. For some individuals, the symptoms can start after only 3 to 6 hours and last for an entire week.

Common symptoms include anxiety, insomnia, irritability and nausea. To reduce the severity of the symptoms, rather than quit caffeine reduce it gradually.

Caffeine sensitive people

Some of the negative effects of caffeine, such as the increased risk of heart attack, are associated with the level of sensitivity to caffeine and this is dependent on the genes.

Fast caffeine metabolizers not only don’t experience some of these negative effects but may benefit from moderate caffeine consumption, especially if taken in form of coffee.

Allergic reaction to caffeine

Cases of allergic reactions to caffeine are very rare , however, caffeine should be avoided altogether in these cases.

This article lists the side effects of caffeine.

Caffeine and loose stools

Some people experience increased bowel movements after a coffee. A study suggested that about 30% of people are affected and around 60% of them are women. (1)

While it is still not entirely clear what are the mechanisms of laxative effects of coffee, it is assumed that certain components of coffee, such as caffeine, act on the receptors in the stomach and small intestine. This action triggers hormonal and neural reaction that induces colonic movement.

These effects intensify with higher dosage of caffeine which may result in loose stools and even diarrhea.


  • Caffeine may act as laxative, and lead to loose stools in some people.

Effects of caffeine on reflux (GERD) and gastroesophageal health


Many studies show that drinking coffee worsens gastroesophageal reflux, commonly called reflux or heartburn. Tea has a similar effect.

Whether caffeine is to blame is not entirely clear, however.

The original assumption was that caffeine causes the lower esophageal sphincter pressure (LESP) to decrease. In other words, the muscle guarding the delicate tissue of the esophagus from the damaging acids of the stomach would relax, causing the stomach acids to damage the tissue in the esophagus.

Since regular coffee consumption has significantly stronger effects on stomach acid release and GERD symptoms than decaffeinated coffee, it was assumed that caffeine is the causing factor.

In addition, studies show that switching to decaffeinated coffee drastically reduces symptoms of GERD.

It was found that LESP decreases significantly more from the various substances created during the process of roasting coffee rather than from caffeine. However, the process of decaffeinating coffee removes some of these substances.

For instance, hydroxytryptamides are substances in roasted coffee that can cause ulcers in the gastric mucosa of rats and may be related to GERD. These substances are reduced by 50% during the caffeine extraction process.

In summary:

  • There is no clear evidence to what extent caffeine is responsible for worsening GERD symptoms
  • Drinking coffee or tea worsens GERD symptoms in some people, however, it is not clear what components cause it
  • Decaffeinated coffee and tea may also cause reflux to worsen, although not as severely as regular coffee, due to removed reflux causing substances

Due to lack of strong evidence of the caffeine effects on GERD, the following are a few tips to reduce the risk of GERD:

  • Figure out your personal tolerance to tea, coffee and other caffeinated beverages. The best method is using elimination diet that will enable you to reduce these beverages to a tolerable level,
  • Switch from regular coffee to decaffeinated coffee, and if symptoms persist, eliminate decaffeinated coffee altogether,
  • Switch from black or green tea to caffeine-free herbal tea.

Functional dyspepsia

Functional dyspepsia is a functional gastrointestinal condition which manifests with a feeling of early satiety and fullness after eating and pain in the stomach area related to meal. (2)

Caffeine is associated with functional dyspepsia.

Obesity and indigestion

Drinking caffeinated beverages can cause indigestion in obese individuals. (3)

See more on obesity statistics for: U.S., UK, and global.

Gastric ulcers

Studies on gastric ulcers are conflicting. Some studies show an association with intake of caffeine, others no such link. (4)


  • Caffeine may contribute to worsening of GERD symptoms, but not to the extent that was once thought.
  • The latest studies show that other components of coffee and tea have more significant effects on GERD.
  • Decaffeinating of coffee removes a large portion of those components, reducing the effects of GERD.
  • Studies on caffeine effects on stomach ulcers are conflicting.

Effects of caffeine on blood pressure

Caffeine raises blood pressure when the tolerance to caffeine is not complete.

If caffeine is taken after a period of abstinence, individuals experience a slight rise in blood pressure.

However, this increase in blood pressure reduces or totally disappears with continuous caffeine intake of the same dosage.

Half of the individuals with a moderate to high intake of caffeine will build tolerance to caffeine within 5 days and their blood pressure will return to normal. The other half, on the other hand, will continue with slightly elevated blood pressure, even if they are used to consume a moderate to high amount of caffeine daily.

The increase in blood pressure is small, of about around 3mm Hg. Nevertheless, these changes may negatively affect individuals at high risk of cardiovascular diseases.

Also regular intake of caffeine by people prone to hypertension may be harmful and is not advisable.

The effects of caffeine on the cardiovascular system is due to the blocking of adenosine receptors and the inhibition of phosphodiesterases. (5, 6)


  • Some people may experience a slight elevation in blood pressure when drinking caffeinated drinks.
  • Avoid regular caffeine intake if you are prone to hypertension or highly caffeine sensitive.

Effects of caffeine on heart health

Risk of heart attack

Health effects of caffeine on the heart health depends if you are a slow or a fast metabolizer.

Slow metabolizers have an increased risk of non-fatal heart attack, if they drink 2-3 cups of coffee daily (200-300mg caffeine). Those who drink 4 cups of coffee per day have a four times greater chance of having a heart attack compared with those who drink about one cup (100mg caffeine) daily.

For fast metabolizers, on the other hand, caffeine improves heart health and reduces the risk of heart attack.

Altered heartbeat

Very high doses of caffeine may lead to atrial fibrillation – disturbance in the heart rhythm. These dosages depend on the individual’s ability to handle caffeine.

Such reactions are mostly common in high concentrations of caffeine from caffeine supplements or synergic effects of caffeine and other substances contained in energy drinks. (7, 8)


  • If you are a slow metabolizer, drinking two or more coffees per day may increase the risk of a heart attack.
  • High caffeine intake may cause heartbeat disturbances especially from caffeine supplements and a high intake of caffeinated energy drinks

Caffeine and anxiety

Continuous usage of caffeine, dependence on caffeine, having an overdose of caffeine or stopping caffeine intake after long-term use may induce symptoms of anxiety. (9)

These symptoms may also occur while you are still developing tolerance (when adjusting to an increased dosage) but are only temporary.

However, when you are on the same dosage for a while and the symptoms continue, it means that you are taking more caffeine than you can handle.

How does it work?

  • Caffeine blocks the receptors of a neurotransmitter called adenosine that has calming effects. By blocking it, it doesn’t allow adenosine to act on your body
  • Caffeine also induces the production of adrenaline

A high caffeine intake have profound effects on adenosine and adrenaline, causing anxiety, jitteriness, stress and nervousness.

The dosages leading to these effects are highly dependent on each individual sensitivity level to caffeine. It may occur with one coffee in sensitive individuals or may required 12 coffees in high caffeine metabolizers.

If you experience these symptoms then you have exceeded the limit of caffeine you can handle. In this case reduce your caffeine intake to the point that you don’t experience these unwanted symptoms.

Caffeine-induces anxiety disorder is a recognized condition by the American Psychiatric Association.


  • Caffeine intake leads to anxiety and nervousness and is dosages dependent.
  • The dosage varies greatly depending on sensitivity to caffeine of each individual.
  • Since there is no caffeine dosage that applies to everyone, you need to find your own limit by reducing caffeine intake until the anxiety symptoms disappear.

Caffeine and dehydration

Caffeine has diuretic properties. When taken in high amounts, especially in a tablet form, it causes the kidneys to increase the production of urine. High doses of more than 360mg of caffeine increase urine production. (10)

However, when taken as coffee, tea, or other caffeinated drinks, such as soda or energy drinks, the contribution of the liquid negates the diuretic effects of caffeine. Caffeinated drinks can, therefore, be considered as being hydrating.

For most individuals, the diuretic effects of caffeine get reduced or completely disappear with the increased tolerance to caffeine (within 1-5 days). This means that after a period of caffeine abstinence, a dose of caffeine may cause various effects on the body, including increased urination, but after becoming accustomed to regular caffeine intake, these symptoms disappear.(11)

If caffeine is consumed before exercise, its’ diuretic properties are negated compared to having caffeine followed by rest.

Please be aware that some studies that have shown the diuretic effects of energy drinks didn’t take into account the caffeine tolerance factor and, therefore, are flawed.


  • Consuming caffeine without liquid may cause more urination
  • Regular caffeine users experience minimal or no effect on urination
  • Caffeine before exercise doesn’t cause increased urination
  • You may urinate more after a caffeinated drink if you are already well hydrated

Effects of caffeine on sleep

Although sleep quality of many coffee (or other caffeinated beverage) drinkers doesn’t get affected by low to moderate caffeine intake, a higher intake can affect sleep quality, especially when taken in the afternoon.

Sleep disturbances include poor sleep quality, reduced sleep time, more time taken to fall asleep and sleep efficiency.

Generally, higher caffeine intake is associated with longer time to fall asleep. (12, 13)

Around 90% of the adult population in the United States consumes caffeine in the afternoon (between noon and 6pm), and 68.5% in the evening (6pm-midnight). On average, adults consume about 329 mg (+- 180mg) of caffeine per day.

For moderate caffeine users (400mg of caffeine/day) sleep disturbances occur with a dosage consumed up to 6 hours before sleep time.

Therefore, it is recommended to abstain from caffeinated drinks for a minimum of 6 hours before sleep time (on average, it should be avoided after 5pm). (14, 15)

Here are some practical tips of what you can do:

A good method of adjusting the caffeine intake to improve your sleep quality is to start a “caffeine intake diary”, similar to what some dieticians use to monitor dietary habits.

In the diary you can record your daily caffeine intake and your sleep pattern. For one week record each caffeinated drink, including amount and time you had it.

Then record how long it took you to fall asleep, whether you woke up in the middle of the night and other sleep disturbances.

The following week eliminate all caffeinated drinks and foods (e.g. chocolate) within 6 hours of going to bed. Record you sleeping patters for about one to two weeks.

If these symptoms persist, reduce caffeine intake and/or have it earlier. Continue until all the symptoms disappear.

Through this method you will discover what is the ideal caffeine pattern intake for you.


  • If you experience sleeping difficulties, it may be due to caffeine
  • If you are caffeine sensitive, avoid caffeinated beverage in the afternoon and if you continue having trouble sleeping, reduce the daily intake and monitor your sleep quality.
  • If you generally handle caffeine well, experiment by avoiding caffeinated drinks within 6 hours before going to sleep.

Effects of caffeine on fertility and risks of miscarriage

Current guidelines recommend limiting caffeine consumption to a maximum of 200mg daily (e.g. 2 coffees). (16) However, this vary between individuals and in some cases 200mg per day may be too much.

Caffeine rapidly crosses the human placenta and reaches a similar concentration in the fetus. (17)

If you are a slow metabolizer and consume 2-3 coffees per day, the risk of infertility and miscarriage increases. (18) An increase of 100mg of caffeine intake per day (equivalent to 1 cup of coffee) is associated with a 7% higher risk of loss of pregnancy (18), and a 13% higher risk of a lower birth rate. (19)

A study showed that an amount of caffeine below 300 mg per day (around 1-2 coffees or 4 cups of tea) doesn’t increase the risk of spontaneous abortion, pregnancy complications, reduce fertility, or have an adverse neurodevelopmental effect.

However, this study doesn’t take into account the caffeine sensitivity aspect, which means that its results may be not applicable to slow metabolizers.

Studies on infertility show that large amounts of caffeine may increase the risk of infertility. Small amounts, however, have no impact. Therefore, when planning to get pregnant is recommended to decrease the caffeine intake to no more than 300mg per day. (20, 21)


  • Large doses of caffeine may increase the risk of infertility
  • Pregnant women who are slow caffeine metabolizers should minimize or avoid caffeine intake to prevent miscarriage

Effects of caffeine on Fibrocystic Breast Disease

Caffeine consumption is associated with a higher risk of fibrocystic breast disease in women. This association is higher for women at a higher risk of developing breast cancer.

Daily caffeine amounts of between 31 and 250mg increase the risk by 1.5 times and amounts of over 500mg by 2.3 times. (22)


  • Caffeine consumption increases the risk of fibrocystic breast disease in women in high breast cancer risk group.

Effects of caffeine on insulin sensitivity

Caffeine consumption can decrease glucose uptake and insulin sensitivity. (23, 24)

Caffeine and menopause

Caffeine use leads to more hot flashes and night sweats in postmenopausal women. (25)

Effects of caffeine on seizure disorders

Caffeine supplements may lower the seizure threshold in individuals with seizure disorders, potentially inducing new seizure occurrences. (16)

Effects of caffeine on energy levels

Caffeine affects our hormonal balance and energy levels in various ways. It causes sleep disturbances, increased energy and alertness, mood changes, and alterations to our physical and mental activity patterns. 

It is common sense: being deprived of sleep, will make you sleepy, being excessively active will make you tired, releasing too much adrenaline will have a toll on your hormonal balance.

These reactions to caffeine cannot go forever without some counter effects. After a few hours or on the next day when the effects of caffeine wear off, depending on sensitivity levels, the rebound effects kick in.

This is when we start feeling fatigued, our mood drops and our energy level considerably lowers.

To avoid this rebound effects, we usually take more caffeine and the vicious cycle continues.

Establishing an optimal regular caffeine intake is important to avoid these unwanted effects.


  • Avoid the shifts in your energy levels by establishing the optimal level of caffeine intake.
  • Being tired and fatigued may indicate that your caffeine intake is throwing you off balance.

Caffeine contraindications – medications

Caffeine increases the activity of the hepatic cytochrome P450 (CYP1A2).  This is a group of enzymes that break down not only caffeine or alcohol but also various medications. This means that an increased activity of CYP1A2 may impact the potency of some medications.

On the other hand some medications may reduce the activity of CYP1A2, decreasing the break-down and elimination of caffeine, therefore causing adverse health effects from an excess of caffeine. (26, 27)

Consult your doctor about the amount of caffeine you should be consuming daily when on medication.

For the full list of drugs that interact with caffeine click here.


  • A group of enzymes in the liver break down various drugs including caffeine, which means that taking caffeine together with those drugs may impact their effects.
  • If you are on medication, consult your doctor about the amount of daily caffeine intake.


You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.


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