Effects of Caffeine on the Human Body

Caffeine is a mild stimulant of the central nervous system (1). It may help prevent sleepiness in some people (1). It may slightly increase the effect of analgesics, such as aspirin or acetaminophen (43). It slightly widens vessels, which may help in migraine, so it is added to certain migraine medications (3).

Caffeine Side Effects

Short-Term Side Effects

Some people may experience caffeine side effects after as little as 2-3 cups of coffee:

  • Anxiety, restlessness, insomnia
  • Dry mouth, thirst
  • Increased heart rate
  • Heartburn, stomach upset, nausea, vomiting, diarrhea (in larger doses, caffeine acts as a laxative) (41)
  • Slight increase of the heart rate and breathing frequency
  • Transient increase of blood pressure (up to 15 mm Hg) (10)
  • Jitteriness
  • Aggravation of the premenstrual syndrome (PMS) (7)
  • Aggravation of irritable bowel syndrome (IBS); it may aggravate diarrhea (8)
  • Aggravation of the existing urinary incontinence (caffeine may increase the urge of urination) (42)
  • Allergic reaction with rash, itchy skin, swollen face and lips (6)
  • Caffeine acts as a slight diuretic–it stimulates urine secretion–, but it does not likely cause dehydration (2,42).
  • (References 1,4,5,6)

Acute Caffeine Intoxication or Overdose

Some people may have symptoms of caffeine intoxication after two cups of coffee (250 mg caffeine), many people after about six cups (600 mg caffeine), but some do not have any symptoms even after 900 milligrams of caffeine (2).

Possible symptoms of caffeine overdose:

  • Headache
  • Facial flushing
  • Nervousness, feeling of fear, depression or even hallucinations
  • Hyperventilation associated with tingling fingertips and toes
  • Hand shaking
  • Pounding heart
  • Fever
  • Dilated pupils
  • Ringing in the ears (tinnitus)
  • Rare complications include: hyperglycemia, hypokalemia, heart attack, stroke, muscle disintegration (rhabdomyolysis)
  • Death from caffeine is rare; about 10 grams of caffeine may be lethal for adults.
  • (References 2,6,7,9,41)

Chronic Caffeine Side Effects

  • Chronic caffeine intoxication or caffeinism (after ingestion of more than 1,000 mg caffeine or about 10 cups of coffee per day) with anxiety, tingling in the fingers and around the mouth, agitation, abdominal pain, anorexia, diarrhea, palpitations, tremor (13,14)
  • Depression (7)
  • Increased risk of fibrocystic breast disease (7)
  • Aggravation of restless leg syndrome (11)
  • Aggravation of symptoms of schizophrenia (12).

Caffeine Withdrawal Symptoms

Withdrawal symptoms typically appear 12-24  hours after the last caffeine dose in individuals who consume caffeine at least three days in the row (16); in some individuals, withdrawal symptoms may appear in as little as 3 hours and may last for a week (21). Main symptoms include headache, restlessness, fatigue, apathy, weakness, flu-like symptoms with stuffy nose (15).

Is Caffeine Good or Bad For You — Caffeine Tolerance

Many people quickly (in few days of regular consumption) develop tolerance to both beneficial effects (alertness) and harmful effects (e.g increase of blood pressure, insomnia) of caffeine (28,29).

How much caffeine is in a cup of coffee or tea?

TABLE: Caffeine content of coffee, tea, coke, energy drinks, chocolate (2,17,18,19,20,21,28)

SOURCE AMOUNT OF CAFFEINE in milligrams (mg)
Espresso, restaurant style (1 oz = 30 mL) 40 (30-90)
Instant (8 oz = 1 cup = 240 mL) 70 (30-170)
Brewed, Arabica (8 oz = 1 cup = 240 mL) 100 (70-120)
Fast-food-size coffee (16 oz = 2 cups = 480 mL) 125 (100-330)
Herbal and fruit tea 0
Green, black, white, oolong tea (8 oz = 1 cup = 240 mL) 30 (15-110)
Cola (including diet coke), soda, root beer; caffeinated (12 oz can = 355 mL) 40 (30-120)
1 can (4-16 oz = 120-480 mL) NOTE: Smaller can does not necessarily mean less caffeine 80 (30-350)
Milk chocolate (1 oz = 28 g) 6
Dark chocolate, 70-85% cacao (1 oz = 28 g) 23
Dark chocolate-coated coffee jelly beans (28 pieces = 40 g) 335

Medicines and Herbal Products Containing Caffeine

Certain prescription or over-the counter medicines, such as analgesics, medications for migraine, diuretics, weight-loss pills, dietary supplements and stimulants may contain up to 400 milligrams caffeine per tablet or capsule (22,23). Certain herbal products containing yerba mate, guarana, green tea extracts and kola nut, contain caffeine. Consult your doctor before using any medication containing caffeine.

How much caffeine is safe during pregnancy?

Exact effects of caffeine on the growing baby are still not known, so doctors often advise against consuming caffeinated beverages during pregnancy. Many doctors recommend women to limit caffeine intake to 200 mg caffeine per day (about two cups of coffee; four cups of tea; five 12 oz cans of cola) to avoid eventual harms (30,31,38).

Risks of Caffeine Intake in Early Pregnancy (First Trimester) and in Third Trimester

  • According to one Swedish study from y. 1998, caffeine intake greater than 100 mg per day by nonsmoking women in the first trimester increases the risk of spontaneous abortion (24).
  • According to one y. 2003 study in Norway, caffeine consumption in the third trimester was associated with lower birth weight, especially in boys (33).
  • Heavy caffeine consumption (at least 400 mg or 4 cups of coffee per day) throughout pregnancy was associated with increased risk of sudden infant death syndrome (SIDS) (40).
  • Some other studies have found no association between caffeine consumption and miscarriage (42).

Caffeine, Stunt Growth and Low Birth Weight

The results of studies about the effect of caffeine intake during pregnancy on birth outcomes are conflicting. Below, few studies are mentioned:

  • According to one large study performed in the UK in y. 2008, caffeine intake as low as 100 mg per day was associated with increased risk of retarded growth of the babies (25).
  • In one large U.S. study from y. 1986, daily intake of 150-300 mg caffeine was associated with about twice increased risk of low birth weight, and intake of more than 300 mg caffeine with more than 4 times increased risk of low birth weight in the babies (26).
  • In one study, heavy smoking, alcohol intake and caffeine intake greater than 400 mg per day throughout pregnancy was associated with low birth weight (27).
  • Some studies have found no significant difference in birth outcomes between mothers who consumed caffeine and those who did not (36,39).

Caffeine During Pregnancy and Irritability and ADHD in Children

  • When mothers regularly consume caffeine during pregnancy, babies can have withdrawal symptoms, such as irritability and vomiting, lasting for few days after delivery (44).
  • One large study in the Netherlands that ended in y. 2012 has shown no association between caffeine intake during pregnancy and hyperactivity or other behavioral problems in children (32).

Caffeine Intake and Birth Defects

Two studies in the U.S. (y. 2009 and 2011) did not reveal any association between caffeine intake during pregnancy and cleft palate or other birth defects in the babies (34,45).

Caffeine Is Pregnancy Category C

Caffeine is a pregnancy category C drug, which means that in animal studies, caffeine had adverse effects on the fetus, but no well controlled studies in humans have been done (47). This means that harmful effects of caffeine on the human fetus have not been firmly proven, but they currently cannot be ruled out.

Caffeine Intake and Infertility

One large study in Denmark (y.2007-2010) (35) and one 2010 study in Saudi Arabia (37) and some other studies (42) have found no significant association between caffeine intake and fertility.

Conclusion: Facts and Myths


Fact 1: Caffeine can get across the placenta and increase baby’s heart rate (46).

Fact 2: Some, but not all, studies have found association between caffeine consumption during pregnancy, miscarriages and and low birth in the babies.


Myth 1: Caffeine during pregnancy is always harmful for the baby. The fact is that not all studies have found harmful effects of caffeine on the babies.

Myth 2: Caffeine leads to irritability and ADHD (Attention Deficit Hyperactivity Disorder) in the newborn child. This association has not been proven.

  1. What is caffeine?  (betterhealth.vic.gov.au)
  2. Caffeine is a mild diuretic  (fda.gov)
  3. Caffeine is vasodilator  (nih.gov/pubmed)
  4. Caffeine side effects  (ncbi.nlm.nih.gov/pubmed)
  5. Caffeine effects and side effects  (nih.gov/pubmed)
  6. Caffeine information  (drugs.com)
  7. Caffeine and premenstrual syndrome  (drugs.com)
  8. Caffeine and gastrointestinal tract  (nih.gov/pubmed)
  9. Caffeine intoxication  (nih.gov/pmc)
  10. Caffeine and high blood presure  (nih.gov/pubmed)
  11. Caffeine and restless leg syndrome  (ninds.nih.gov)
  12. Caffeine and schizophrenia  (nih.gov/pubmed)
  13. Chronic caffeine intoxication  (emedicine.medscape.com)
  14. Psychological and psychiatric effects of caffeine  (rcpsych.org)
  15. Caffeine withdrawal symptoms  (nih.gov/pmc)
  16. Caffeine withdrawal onset  (springer.com)
  17. Caffeine levels in coffee, tea, cola, energy drinks  (mayoclinic.com)
  18. Caffeine content of beverages  (energyfiend.com)
  19. Foods and drinks high in caffeine  (nutritiondata.self.com)
  20. Amount of caffeine in tea  (choiceorganicteas.com)
  21. Withdrawal symptoms onset and end  (emedicine.medscape.com)
  22. Caffeine content in supplements, drugs and OTC medications  (energyfiend.com)
  23. Dietary supplements with caffeine  (ars.usda.gov)
  24. Caffeine and spontaneous abortion in the first trimester  (nih.gov/pubmed)
  25. Caffeine in pregnancy and retarded fetus growth  (bmj.com)
  26. Caffeine and low birth weight  (oxfordjournals.org)
  27. Alcohol, smoking, caffeine and low birth weight  (bmj.com)
  28. Caffeinated drinks and foods: diet Coca Cola, Pepsi, Mountain dew, Dr. Pepper, Surge, Starbucks coffee, chocolate  (fda.gov)
  29. Caffeine tolerance  (pharmrev.aspetjournals.org)
  30. Is caffeine good or bad for you when you are pregnant?  (marchofdimes.com)
  31. Maximal amount of caffeine in pregnancy  (babycenter.in)
  32. Maternal caffeine intake and child behaviour  (nih.gov/pubmed)
  33. Caffeine intake in the third trimester and fetal growth  (nih.gov/pubmed)
  34. Caffeine intake during pregnancy and cleft palate  (nih.gov/pubmed)
  35. Caffeinated beverages and soda intake and fertility  (nih.gov/pmc)
  36. Effect of caffeine restriction during pregnancy on birth outcomes  (nih.gov/pubmed)
  37. Caffeine consumption in pregnancy and fertility  (nih.gov/pubmed)
  38. Opinion of American College of Obstetricians and Gynecologists (ACOG) about caffeine in pregnancy  (nih.gov/pubmed)
  39. Caffeine and pregnancy outcome   (nih.gov/pubmed)
  40. Caffeine and sudden infant death syndrome  (nih.gov/pmc)
  41. Caffeine overdose symptoms and treatment  (nih.gov/pubmedhealth)
  42. Caffeine and women’s health  (foodinsight.org)
  43. Caffeine in common painkillers  (nih.gov/pubmed)
  44. Caffeine withdrawal in infants  (nih.gov/pubmed)
  45. Caffeine and birth defects  (nih.gov/pubmed)
  46. Caffeine and fetal heart rate  (nih.gov/pubmed)
  47. Caffeine pregnancy category  (drugs.com) [/ref]