Harmful effects of coffee: Why too much caffeine is bad?

Coffee is a common drink made from the beans of Coffea arabica bush. Most articles on the web focus on its health benefits but coffee and caffeine also have some side effects and may interact with medical drugs. In this article, we shall take a detailed evidence-based look at side effects and interactions of coffee. You will also learn about the maximum safe daily dose of caffeine you should not exceed.

An average American drinks about 2 to 4 cups of coffee a day, which translates to 300 milligrams of caffeine a day.

Drinking coffee has many beneficial effects on your health.

Caffeine relieves pain, reduces the risk of developing type 2 diabetes, as well as other severe disorders, such as such liver cirrhosis, Alzheimer's disease, Parkinson's disease, or depression (1, 2, 3, 4, 5).

Studies also suggest that people who drink coffee have a lower risk of heart disease, multiple sclerosis, and colorectal cancer (6, 7, 8).

Side effects of coffee and caffeine

Unfortunately, coffee has some harmful effects that we should bear in mind.

Here's an overview of the most important adverse effects of caffeine:

Caffeine may increase cardiovascular risk in patients with hypertension

Drinking coffee may increase the stiffness of the aorta, which is a significant risk factor for cardiovascular disease.

In healthy people, the aorta is elastic and adapts to the pressure under which the heart pumps the blood into the body. One of the primary causes of hypertension (high blood pressure) is "stiffening" of the aorta. When the aorta loses elasticity, there is an increase in blood pressure in the heart, which translates to a higher risk of cardiovascular disease.

In one randomized, placebo-controlled study, one group of participants drank coffee containing 250 mg caffeine while other participants got a placebo drink.

In participants who drank coffee, the aorta became stiffer for 3 hours, and they had a higher blood pressure than those in the placebo group (9).

Another study, conducted by Mayo Clinic, suggests that caffeine in energy drinks increases blood pressure in young people under 40 years of age.

The most significant increase in blood pressure was found in people who were not used to caffeine (drank less than 160 mg of caffeine per day).

But the blood pressure was higher even in people, who regularly consumed more than 160 mg of caffeine a day (10).

Coffee increases the risk of myocardial infarction in young people with hypertension

One Italian study on 1200 participants indicates that caffeine increases the risk of myocardial infarction (heart attack) in people with untreated mild hypertension (systolic blood pressure between 140-159 mm Hg and/or diastolic pressure between 90-99 mg Hg) (11).

Caffeine may increase the risk of gout attacks

Gout is an inflammatory joint disease, caused by uric acid accumulation in joints and tendons.

The primary symptom of gout is the pain in the joints (mostly big toe joint).

The number of people with gout is growing.

Most studies that examined the effect of drinking coffee on the bottom confirm that regular coffee drinking reduces the risk of gout (12).

But if you tend to binge coffee drinking (for example, you do not drink any coffee for 2 days and then give it 3 per day) the risk of getting a seizure of days increases (13).

Caffeine increases the risk of fibrocystic breast disease

Drinking coffee can be associated with a fibrocystic breast disease. A common symptom of this disease is painful and swollen breasts.

The fibrocystic breast disease may turn into benign breast cancer (especially fibroadenoma).

A study involving 634 women suggests that women who consumed between 31 - 250 mg caffeine daily had 1.5 times higher risk of fibrocystic breast disease than women who did not drink any coffee (14).

Women who drank more than 500 mg of caffeine daily had the risk of fibrocystic breast disease 2.3 times higher than those who did not drink any coffee.

Caffeine contributes to incontinence and headache

Caffeine is one of the factors contributing to incontinence or headaches.

A study in Alabama found that women who drink more than 329 mg of coffee daily (corresponding to about 3-4 cups) have a 70% higher chance of urinary incontinence (15).

Studies also suggest that drinking coffee aggravates dehydration (lack of fluids in the body) because caffeine has diuretic effects (promotes urination) (16).

Therefore, the FDA recommends that people always drink plain still water with coffee to excessive fluid loss and dehydration.

Some studies also suggest that caffeine may increase the risk of chronic headaches (17).

Caffeine may impair sleep quality

Caffeine stimulates your nervous system and promotes alertness.

If you drink coffee in the evenings or just before bedtime, you may have trouble sleeping.

One study compared the effect of different beverages on the quality of sleep (18).

Study participants were either drinking hot water, decaffeinated coffee or caffeinated coffee 30 minutes before going to bed.

The results suggest that while hot water and decaffeinated coffee do not affect the quality of sleep, caffeine may cause insomnia.

Caffeine worsens fertility in women and increases the risk of miscarriage

Drinking coffee may negatively affect fertility in women

Analysis of several studies suggests that caffeine may be one of the factors associated with infertility (19).

Another study suggests that drinking more than two caffeine drinks per day increases the risk of miscarriage (20).

Therefore, pregnant and breastfeeding women should not consume more than 200 mg caffeine per day, which translates to 2 common cups of coffee a day (21).

Caffeine may cause palpitations and tremors

A study on 4,558 people who consumed 240 mg of caffeine a day or more suggests that drinking coffee may be associated with anxiety, palpitations (when you hear your heart beating) and tremor (22).

If these symptoms occur, limit the daily amount of coffee or other caffeinated beverages.

If the symptoms do not clear off after lowering your caffeine consumption, seek a prompt medical attention.

Caffeine may aggravate anxiety and depression

In reasonable amounts, coffee has stimulating effects, but excessive caffeine consumption may cause anxiety and depression or aggravate their symptoms.

Studies suggest that daily consumption of 250 mg of caffeinated beverages results in euphoria while a daily dose of 500 mg increases irritation (23).

Caffeine exacerbates the symptoms of menopause

Menopause is the end of the fertility period in a woman's life. Due to hormonal changes in the body, menopause is associated with some common issues, such as red flashes and other vasomotor symptoms.

Studies suggest that caffeine worsens vasomotor symptoms of menopause (24).

Caffeine increases the risk of bone fractures

Coffee and other caffeinated drinks speed up the release of calcium from the bones and its elimination from the body via the kidneys (urine).

Calcium is necessary for healthy bones, so drinking too much coffee may aggravate the symptoms of osteoporosis and increase the risk of fractures.

An analysis of 10 studies with over 214,000 participants indicates that for every cup of coffee a risk of fractures increases by 3.5% (25).

Coffee decreases the absorption of iron

Iron is an essential element that your body needs to make red blood cells, which distribute oxygen to tissues.

Coffee is a rich source of chlorogenic acid.

This phenolic compound is a potent inhibitor of non-heme dietary iron absorption (26).

Insufficient iron absorption may lead to iron deficiency, which causes anemia (lack of red blood cells).

On the other hand, iron deficieny reduces the risk of type 2 diabetes.

Coffee and digestive issues

Drinking coffee on an empty stomach increases the production of hydrochloric acid in the stomach, which may translate to many digestive problems, such as heartburn, irritable bowel syndrome, bloating or gas (27).

Excessive consumption of coffee and other caffeinated beverages can also cause diarrhea because caffeine makes your gut move faster (peristalsis).

However not all studies came to the same conclusions, and some suggest that coffee has no impact on GERD (gastroesophageal reflux disease) (27).

Therefore, more research is necessary before we could make any recommendations.

Coffee and acrylamide

Acrylamide is a hazardous substance that may cause cancer.

It is produced while roasting coffee beans at high temperatures.

The darker the coffee, the more acrylamide it usually contains (28).

Coffee is one of the most significant dietary sources of acrylamide.

If you drink coffee sparingly, you do not have to worry about acrylamide, but if you drink too much, you may be at risk.

Coffee and glaucoma

Blood pressure-enhancing effects of coffee manifest not only in the heart and the aorta but also in other organs.

Studies suggest that excess of caffeinated coffee may temporarily increase intraocular pressure, which leads to a worsening of glaucoma symptoms (29).

If you have glaucoma, do not drink more than one cup of coffee a day or opt for decaffeinated coffee, instead.

Coffee interactions with medicines

Caffeine in coffee and other caffeinated beverages can interact with certain drugs and affect their effects.

Because coffee stimulates the nervous, cardiovascular and digestive system, it is necessary to pay particular attention to the drugs that affect these systems.

Coffee is healthy but in excess it may have some harmful side effects

Here is an overview of medical drugs, which may interact with coffee:

Ephedrine and other stimulants

Because both coffee and ephedrine are stimulants and their simultaneous use makes their effects stronger.

Drinking caffeinated coffee and taking ephedrine at the same time can cause palpitations, arrhythmias, and cause cardiac arrest and death (30).

Therefore, never use ephedrine and caffeine-containing drinks, foods, medicines or supplements at the same time.

Other stimulants that interact with caffeine include diethylpropion, epinephrine, phentermine, pseudoephedrine (Sudafed), etc.

Adenosine

Coffee blocks adenosine (P1) receptors in the brain that are responsible for drowsiness and fatigue. 

Therefore, it can impair the effects of drugs that act on adenosine receptors, such as Adenocard (Adenosine) (31).

Doctors use Adenocard on patients, who undergo a cardiac stress test.

Do not drink any coffee or use any other caffeine-containing substance at least 24 hours prior your cardiac stress test or prior taking Adenosine medication.

Seizure medications

Coffee may slow or accelerate the excretion of some medications to cramps from the body and intensify or weaken their effects (32).

Drinking coffee at the same time as taking these medicines can lead to a worsening of convulsions.

These drugs include, for example, Rilutek (Riluzole), Valproate, Tiagbine, Phenytoin, Ethosuximide, Phenobarbital, Felbamate, Carbamazepine (Tegretol), etc.

Talk to your healthcare provider before drinking coffee while on these medications.

Histamine receptor antagonists

Medicines that block histamine receptors (antihistamines) slow down the excretion of caffeine from the body. Therefore, side effects such as heart rhythm disturbances, headache, nausea, and the like may occur when taken with coffee.

Cimetidine (Tagamet) is an example of antihistamines, which may interact with caffeine (32).

Drugs for depression, anxiety, and other mental disorders

Caffeine interacts with monoamine oxidase inhibitors (MAOIs), mood stabilizers, selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants, antipsychotics and other psychiatric drugs.

Drinking coffee with these medical drugs can both increase the caffeine side effects or reduce the efficacy of these medicines.

Examples of psychiatric drugs, which may interact with caffeine include Phenelzine (Nardil), Tranylcypromine (Parnate), Lithium, Sertraline (Zoloft), Clozapine (Clozaril), etc.

Anticoagulants and antiplatelet drugs

Caffeine slows blood clotting.

If you drink coffee and take medication that slows down blood clotting, its effect is increased, which may cause bruising and bleeding (32).

Examples of anticoagulants that interact with caffeine are aspirin, clopidogrel (Plavix), diclofenac (Voltaren, etc.), ibuprofen (Advil, Motrin), naproxen (Anaprox, Naprosyn, etc.), enoxaparin, dalteparin, heparin, warfarin ), ticlopidine (Ticlid), etc.

Some drugs for respiratory diseases

Asthma drugs (Beta-adrenergic agonists) also stimulate heart activity.

Since caffeine has the same effects on the heart, drinking coffee and concomitant use of asthma medications may multiply the activity of these drugs, which can cause cardiac problems, such as palpitations and even a heart failure (32).

Medical drugs for asthma, COPD and other respiratory diseases, which may interact with caffeine include albuterol (Proventil, Volmax, Ventolin), theophylline, metaproterenol (Alupent), terbutaline and isoproterenol (Isuprel), etc.

Contraceptive drugs and other hormonal medicines

Birth control pills and other hormonal drugs (estrogens, estradiol, etc.) slow down caffeine excretion from the body. Concomitant use of coffee and contraceptives may cause side effects associated with caffeine overdose, such as fast heartbeat, headache, jitteriness, etc (32).

Alcohol and drugs for treatment of alcohol abuse

Alcohol slows caffeine excretion from the body. Likewise, some medicines used to treat alcohol dependence (such as Antabuse (disulfiram) act in a similar way (32).

Drinking coffee along with alcohol or concomitant use of disulfiram may cause side effects associated with excessive caffeine intakes, such as cardiac problems, nausea, or a headache.

Some medicines for diabetes

Some studies suggest that caffeine may lower the blood sugar level and increase it. Because antidiabetes drugs decrease blood sugar, coffee can impair their effects (32).

If you drink coffee while taking diabetes medications, monitor your blood sugar levels carefully and, if necessary, seek an immediate medical attention.

Antidiabetes drugs that interact with caffeine include glimepiride, glyburide (Glynase, Micronase, etc.), insulin, pioglitazone (Actos), rosiglitazone, chlorpropamide, glipizide, metformin and many others.

Some medicines for heart diseases

Some medicines to treat heart disease (primarily calcium channel blockers) slow down the excretion of caffeine from the body. Consequently, side effects such as palpitations, rapid heartbeat, headaches, etc. may occur when taken with coffee (32).

Diuretics

Caffeine reduces potassium levels in the body. Because some water pills (diuretics) also lower potassium in the blood, drinking coffee may cause hypokalaemia (lack of potassium in the blood) when taken concomitantly (32).

Diuretics, which may cause hypokalaemia in combination with caffeine, include chlorothiazide, furosemide, etc.

Other medicines

Other medicines that may interact with caffeine include methoxsalen (for the treatment of psoriasis), fluconazole (for the treatment of mycotic infections), flutamide (Eulexin), etc. (32)

How much coffee a day is safe?

It is not easy to determine a safe daily dose of coffee.

Caffeine has different effects on every person, which means that some people may drink more coffee than others without experiencing any symptoms of caffeine overdose.

Some people are also allergic to caffeine, and they should not drink any caffeinated coffee or drink at all.

Causes of allergy to caffeine include skin problems (urticaria, rash, acne, itching), headaches, migraines, anxiety and panic attacks, loss of concentration, swollen tongue or throat, breathing problems, mood disorders, fatigue, rapid heartbeat, palpitations, chest pain, depression, face, hand and foot tingling, muscle pain, nausea, hallucinations, cold/flu symptoms, vision problems, sweating, etc.

If you feel any of the symptoms above, see your doctor immediately.

A 300-400 mg caffeine daily is considered safe for most people, which corresponds to about five espressos, 1 Starbucks Venti coffee, 5 Red Bull cans and 130 oz (3.8 l) of Coke (33).

If you are pregnant or breast-feeding, you should not drink more than 200 mg of caffeine per day (34).

In pregnancy, do not exceed the maximum daily dose of 200 mg caffeine because of the risk of harm to the unborned child.

In excess, caffeine may have teratogenic effects and decrease the weight of the fetus.

Notwithstanding the above, you should probably not drink more than 2-3 cups of coffee a day.

If you are pregnant or breastfeeding, do not drink more than 1 cup of coffee per day.

The bottom line

Coffee has many benefits for your health.

However, its excessive consumption can be harmful because caffeine also has many side effects.

The most common harmful effects of coffee are cardiac disorders (palpitation, rapid heartbeat), headaches, anxiety, depression and also digestive problems, such as diarrhea or IBS.

Excess caffeine also aggravates symptoms of osteoporosis and increases the risk of bone fractures.

Because coffee is a stimulant, it may interact with some other drugs, such as antidepressants, medicines for cardiac diseases or medications for diabetes.

Especially dangerous is the interaction of caffeine with ephedrine, which may be fatal.

Do not drink any coffee with ephedrine-containing pharmaceuticals.

For a general population, a daily dose of caffeine between 300-400 mg of caffeine should be safe.

Pregnant women and nursing mothers should limit their caffeine intake to 200 mg a day to prevent severe damage to the unborn child.

However, if you take any medicines, do not drink coffee and other beverages containing caffeine (such as energy drinks or Coke) without a prior consent of your doctor.