Caffeine

Caffeine is a natural stimulant commonly found in tea, coffee, and cacao plants. It was discovered by a German chemist, Friedlieb Ferdinand Runge, in 1819, who called it “kaffebase” in reference to the substance’s origin from coffee beans.¹ In its pure form, caffeine is a white crystalline powder that tastes very bitter.

Caffeine works by stimulating the brain and central nervous system, helping to stay alert and preventing the onset of tiredness. It achieves this by mimicking adenosine, a neurotransmitter that signals the brain to prepare for sleep, and binding to the adenosine receptors in the brain. Caffeine occupying these receptors prevents adenosine from binding and keeps us awake.²

The consumption of caffeine has various effects on the body. In moderate doses, caffeine can increase alertness, reduce fine motor coordination, cause insomnia, cause headaches and dizziness, speed up the heart rate, and cause dehydration. ³ However, it can also improve certain types of cognitive performance, such as vigilance, alertness, and mood, but may not enhance more complex cognitive functions.⁴

In higher doses, caffeine can lead to a condition known as caffeine use disorder, which is characterised by a persistent desire or unsuccessful effort to cut down or control caffeine use, continued use despite harm, and withdrawal symptoms during abstinence.⁵

Appearance

Caffeine, in its pure form, appears as a white crystalline powder or small white crystals. ⁶

Tastе

Pure caffeine has a very bitter taste, which is often masked in caffeinated beverages with the use of sugars or other sweeteners.

Smell

Pure caffeine is odourless. The aroma associated with caffeinated beverages like coffee is due to other compounds within the beverage, not the caffeine itself.⁶

Caffeine is most commonly ingested orally through beverages like coffee, tea, and energy drinks. It is also present in certain foods like chocolate and some medicines, particularly those for headaches, colds, and staying awake. In medical scenarios, caffeine can be administered intravenously to treat certain conditions, such as apnoea of prematurity in newborns.⁷

The dosage of caffeine can vary widely depending on the individual and the source of caffeine. Here is a general guideline:

• Minimum to feel something: The threshold dose for caffeine’s physiological effects is generally around 10mg. ⁴
• Low dose: A low dose might range from 20mg to 200mg. A typical cup of tea contains about 30-50mg, while a cup of coffee may contain 95-200mg. ³
• Common dose: A common dose for adults could range between 200mg and 400mg per day. This is roughly equivalent to 2-4 cups of coffee.⁴
• High dose: High caffeine doses are generally considered to be above 400mg per day. Consuming large amounts of caffeine can lead to negative side effects such as restlessness, insomnia, heart palpitations, and muscle tremors. ⁸

However, it’s important to note that individual tolerance to caffeine can vary, and these ranges might not apply to everyone.

Exceeding High Dose: Exceeding high doses of caffeine, typically considered to be above 400mg per day for most adults, can lead to caffeine intoxication. Symptoms may include restlessness, insomnia, flushed face, increased urination, gastrointestinal disturbance, muscle twitching, irregular or rapid heartbeat, and periods of inexhaustibility. In extreme cases, it can cause hallucinations, seizures, and even death. ^8,9

Users report feelings of increased alertness, reduced fatigue, uplifted mood, and often a perceived improvement in cognitive performance such as memory and reaction time. ^4,10

Physiological effects include increased heart rate, dilation of the pupils, increased blood pressure, bronchial relaxation, and increased metabolic rate.

The effects of caffeine can begin as quickly as 20 minutes after consumption and typically reach peak levels within 60 minutes of ingestion. ¹⁰

The half-life of caffeine in the human body is typically in the range of 3 to 5 hours. This means that the stimulant effects of a moderate dose can linger for many hours.¹⁰

Overconsumption can lead to insomnia, restlessness, increased heart rate, upset stomach, nausea, and headaches.

Caffeine can lead to physical dependence, and habitual consumers often experience withdrawal symptoms when they attempt to quit. However, the compulsive drug-seeking behaviour associated with serious drug addiction is generally not seen with caffeine use. That being said, caffeine use disorder, characterised by a failure to control use, continued use despite harm, and withdrawal, has been increasingly recognised in the scientific literature.

In the UK, caffeine is legal and is not classified under the Misuse of Drugs Act 1971. It is available without prescription.

Caffeine may interact with various other substances. For example, combining with alcohol can mask the depressant effects of alcohol, leading to an increased risk of alcohol-related injury. Mixing caffeine with other stimulants can exacerbate side effects like heart rate increase and potentially lead to severe cardiovascular problems.³

1. Brock, J. (1999). Just the tonic: a natural history of tonic water. The Lancet, 354, SIV22. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(99)90240-2/fulltext
2. Fredholm, B. B., Bättig, K., Holmén, J., Nehlig, A., & Zvartau, E. E. (1999). Actions of caffeine in the brain with special reference to factors that contribute to its widespread use. Pharmacological Reviews, 51(1), 83-133. https://pharmrev.aspetjournals.org/content/51/1/83
3. Temple, J. L., et al. (2017). The Safety of Ingested Caffeine: A Comprehensive Review. Frontiers in Psychiatry, 8, 80. https://www.frontiersin.org/articles/10.3389/fpsyt.2017.00080/full
4. Nehlig, A. (2010). Is caffeine a cognitive enhancer? Journal of Alzheimer’s Disease, 20(s1), S85-S94. https://content.iospress.com/articles/journal-of-alzheimers-disease/jad091315 
5. Addicott, M. A. (2014). Caffeine Use Disorder: A Comprehensive Review and Research Agenda. Journal of Caffeine Research, 4(3), 114-130. https://www.liebertpub.com/doi/10.1089/jcr.2014.0016
6. Ferré, S. (2008). An update on the mechanisms of the psych stimulant effects of caffeine. Journal of Neurochemistry, 105(4), 1067-1079. https://onlinelibrary.wiley.com/doi/full/10.1111/j.1471-4159.2007.05196.x
7. Eiland, L. S., & Gilman, J. T. (2018). Caffeine citrate: A summary of the pertinent information about this drug commonly used to treat apnea of prematurity. Journal of Pediatric Pharmacology and Therapeutics, 23(2), 141-148. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5928420/
8. Juliano, L. M., & Griffiths, R. R. (2004). A critical review of caffeine withdrawal: empirical validation of symptoms and signs, incidence, severity, and associated features. Psychopharmacology, 176(1), 1-29. https://link.springer.com/article/10.1007/s00213-004-2000-x
9. Winston, A. P., Hardwick, E., & Jaberi, N. (2005). Neuropsychiatric effects of caffeine. Advances in Psychiatric Treatment, 11(6), 432-439. https://www.cambridge.org/core/journals/advances-in-psychiatric-treatment/article/neuropsychiatric-effects-of-caffeine/7C884B2106D772F02DA114C1B75D4EBF
10. McLellan, T. M., Caldwell, J. A., & Lieberman, H. R. (2016). A review of caffeine’s effects on cognitive, physical and occupational performance. Neuroscience & Biobehavioral Reviews, 71, 294-312. https://www.sciencedirect.com/science/article/pii/S0149763416300690