Tea contains more caffeine than coffee by dry weight. A typical serving, however, contains much less, since less of the product is used as compared to an equivalent serving of coffee. Also contributing to caffeine content are growing conditions, processing techniques, and other variables. Thus, teas contain varying amounts of caffeine. [231]

Caffeine increases intraocular pressure in those with glaucoma but does not appear to affect normal individuals. [135] Global coffee consumption, 2020/21". Statista. Archived from the original on 3 March 2021 . Retrieved 10 March 2021. Caffeine is metabolized in the liver by the cytochrome P450 oxidase enzyme system, in particular, by the CYP1A2 isozyme, into three dimethyl xanthines, [188] each of which has its own effects on the body: a b Introduction to Pharmacology (thirded.). Abingdon: CRC Press. 2007. pp.222–223. ISBN 978-1-4200-4742-4. Archived from the original on 14 January 2023 . Retrieved 25 August 2017.

Key facts

For the general population of healthy adults, Health Canada advises a daily intake of no more than 400mg. [72] This limit was found to be safe by a 2017 systematic review on caffeine toxicology. [73] Children Caffeine was formerly used as a second-line treatment for ADHD. It is considered less effective than methylphenidate or amphetamine but more so than placebo for children with ADHD. [49] [50] Children, adolescents, and adults with ADHD are more likely to consume caffeine, perhaps as a form of self-medication. [50] [51] Enhancing performance Cognitive performance Caffeine is a central nervous system stimulant that may reduce fatigue and drowsiness. [12] At normal doses, caffeine has variable effects on learning and memory, but it generally improves reaction time, wakefulness, concentration, and motor coordination. [52] [53] The amount of caffeine needed to produce these effects varies from person to person, depending on body size and degree of tolerance. [52] The desired effects arise approximately one hour after consumption, and the desired effects of a moderate dose usually subside after about three or four hours. [7] Regular consumption of caffeine may protect people from liver cirrhosis. [129] It was also found to slow the progression of liver disease in people who already have the condition, reduce the risk of liver fibrosis, and offer a protective effect against liver cancer among moderate coffee drinkers. A study conducted in 2017 found that the effects of caffeine from coffee consumption on the liver were observed regardless of how the drink was prepared. [130]

Hillis DM, Sadava D, Hill RW, Price MV (2015). Principles of Life (2ed.). Macmillan Learning. pp.102–103. ISBN 978-1-4641-8652-3. a b c Juliano LM, Griffiths RR (October 2004). "A critical review of caffeine withdrawal: empirical validation of symptoms and signs, incidence, severity, and associated features". Psychopharmacology. 176 (1): 1–29. doi: 10.1007/s00213-004-2000-x. PMID 15448977. S2CID 5572188. Results: Of 49 symptom categories identified, the following 10 fulfilled validity criteria: headache, fatigue, decreased energy/ activeness, decreased alertness, drowsiness, decreased contentedness, depressed mood, difficulty concentrating, irritability, and foggy/not clearheaded. In addition, flu-like symptoms, nausea/vomiting, and muscle pain/stiffness were judged likely to represent valid symptom categories. In experimental studies, the incidence of headache was 50% and the incidence of clinically significant distress or functional impairment was 13%. Typically, onset of symptoms occurred 12–24 h after abstinence, with peak intensity at 20–51 h, and for a duration of 2–9 days. Funk GD (November 2009). "Losing sleep over the caffeination of prematurity". The Journal of Physiology. 587 (Pt 22): 5299–300. doi: 10.1113/jphysiol.2009.182303. PMC 2793860. PMID 19915211.In general, dark-roast coffee has very slightly less caffeine than lighter roasts because the roasting process reduces caffeine content of the bean by a small amount. [229] [230] Tea Some analog substances have been created which mimic caffeine's properties with either function or structure or both. Of the latter group are the xanthines DMPX [210] and 8-chlorotheophylline, which is an ingredient in dramamine. Members of a class of nitrogen substituted xanthines are often proposed as potential alternatives to caffeine. [211] [ unreliable source?] Many other xanthine analogues constituting the adenosine receptor antagonist class have also been elucidated. [212] Caffeine does not appear to be a reinforcing stimulus, and some degree of aversion may actually occur, with people preferring placebo over caffeine in a study on drug abuse liability published in an NIDA research monograph. [114] Some state that research does not provide support for an underlying biochemical mechanism for caffeine addiction. [2] [115] [116] [117] Other research states it can affect the reward system. [118]