Caffeine belongs to a group of naturally occurring compounds called methylxanthines, which also includes theophylline and theobromine. Caffeine, also known by its chemical name 1,3,7-trimethylxanthine, is a psychostimulant that can be consumed in the form of coffee, tea, soft drinks, energy drinks, chocolate, or over-the-counter medication (Hart & Ksir, 2022). Unlike many other psychoactive substances, it is legal and widely available in everyday products, making it one of the easiest stimulants to obtain. Caffeine has been used by many people for centuries, with evidence of use dating back to the Paleolithic period (Barone & Roberts, 1984). Because of its affordability and accessibility, caffeine is commonly used by college students, shift workers, office employees, athletes, and anyone looking to improve alertness or reduce feelings of fatigue. Ferré (2016) explains that caffeine is the most widely consumed psychoactive drug in the world because it effectively increases alertness while having a much lower potential for abuse than stimulants such as cocaine or amphetamines. People, ranging from teenagers to older adults, use this drug to increase their alertness and boost their energy levels.

Xanthines are a chemical class made up of caffeine, theophylline, and theobromine (Hart & Ksir, 2022). Caffeine is most often administered orally, and in medical cases or for neonatal care it can be administered intravenously (Yang et al., 2021). Caffeine is most commonly consumed by mouth through beverages such as coffee, tea, soft drinks, and energy drinks. It is also found in chocolate, caffeine tablets, and certain over-the-counter medications used to treat headaches and migraines. After ingestion, caffeine is rapidly absorbed through the digestive system, and its effects typically begin within about 30 minutes. Approximately two hours after consumption, peak central nervous system effects can be seen (Hart & Ksir, 2022). However, effects can typically be seen within 30 to 60 minutes of absorption (Yang et al., 2021). The bioavailability of caffeine when absorbed orally and intravenously is 100% (Malviya et al., 2023). The route of administration can affect the absorption of caffeine (Hines et al., 2019). Intravenous absorption leads to a faster onset and more intense effects, while oral administration tends to have slower effects (Hines et al., 2019).

Caffeine affects several important areas of the brain that regulate alertness, attention, motivation, learning, and decision-making. One of the primary regions influenced is the prefrontal cortex, which is responsible for planning, concentration, self-control, and higher-order thinking. By increasing activity in this area, caffeine can improve focus and mental performance, particularly when a person is tired. Another important brain region affected is the nucleus accumbens, which is part of the brain’s reward system. This area helps regulate motivation, reinforcement, and goal-directed behavior. According to Ferré (2016), caffeine indirectly increases dopamine activity in the nucleus accumbens by blocking adenosine receptors. Unlike stimulants such as cocaine or amphetamines, caffeine does not produce a large surge of dopamine. Instead, it enhances normal dopamine signaling, allowing people to feel more awake, motivated, and mentally engaged without producing the same high potential for addiction.

Caffeine also affects the striatum, where adenosine A2A receptors interact with dopamine D2 receptors. Blocking these receptors increases movement, alertness, and motivation. In addition, caffeine activates the brain’s ascending arousal system, which plays a key role in maintaining wakefulness and reducing feelings of fatigue throughout the day. Caffeine also impacts the mesolimbic dopamine pathway and the nigrostriatal dopamine pathway (Hsu, Wang, & Chiu, 2010). The mesolimbic pathway is important for reward behaviors (Hart & Ksir, 2022). The nigrostriatal pathway is important for movement (Hart & Ksir, 2022). Hsu, Wang, & Chiu (2010) found that caffeine indirectly increases dopamine signaling when it blocks the adenosine receptors. As a result, dopamine signaling becomes more effective, leading to increased alertness, improved concentration, and greater motivation. These effects explain why caffeine is commonly used to stay awake while studying, working, or driving. This means that when the dopamine signals increase, it causes alertness and locomotor activity. Acute, or short-term, effects of caffeine use are stimulation and alertness. Chronic, or long-term, effects are tolerance, dependence, and withdrawal.

The primary neurotransmitter affected by caffeine is adenosine. Caffeine acts as an antagonist on the adenosine receptor (Yang et al., 2021). Adenosine inhibits neurotransmitters, causing a feeling of calm and sedation (Hart & Ksir, 2022). Caffeine blocks the adenosine receptors, meaning the neurotransmitters cannot send signals to the brain telling it to feel calm and sedated. Caffeine causes the adenosine to decrease its neural firing and synaptic transmission, which increases alertness and reduces feelings of fatigue. As discussed previously, caffeine directly impacts the adenosine receptors and indirectly impacts the dopamine receptors. As a result, dopamine signaling increases indirectly, producing greater alertness, faster reaction times, and improved attention without directly stimulating dopamine release. The effects of caffeine differ depending on whether it is used occasionally or regularly.

Caffeine has many subjective effects. Childs and de Wit (2006) studied the effects of caffeine on non-users to see the effects of caffeine when not influenced by withdrawal. They found that their subjects experienced a more positive mood and were more alert (Childs & de Wit, 2006). Childs and de Wit (2006) also found that the effects were dose dependent, meaning they were stronger at higher doses. However, moderate doses, such as a cup of coffee, produced subjective effects (Childs & de Wit, 2006). An experiment conducted by Harrell and Juliano (2009) found that participants described feelings of nausea, muscle aches, cold or hot spells, jitteriness, rapid heartbeat, blurred vision, and headaches when consuming caffeine. Interestingly, Harrell and Juliano (2009) and Childs and de Wit (2006) both found that caffeine can improve reaction time. It is important to note, though, that there can be variability across individuals, especially for those who have a tolerance, naturally anxious individuals, and those who metabolize caffeine faster or slower. Most people consume caffeine because it helps them feel more awake, focused, and energized. Moderate amounts often improve attention, concentration, reaction time, and mood while reducing feelings of tiredness. These effects make caffeine especially popular among students, shift workers, and individuals performing mentally demanding tasks.

The effects of caffeine depend largely on the amount consumed. Lower doses are generally associated with improved mental performance and increased alertness, while larger amounts are more likely to produce unwanted side effects. According to the textbook, doses of approximately 500 milligrams or more are associated with increased heart rate and other physiological changes. High doses may also cause anxiety, nervousness, restlessness, insomnia, and jitteriness. Caffeine causes a variety of behavioral and physiological effects. Hart and Ksir (2022) found that individuals who regularly had high amounts of caffeine performed poorer on mental tasks. They also found that caffeine can help to relieve headaches and migraines (Hart & Ksir, 2022). Childs and de Wit (2006) found that caffeine increases attention and psychomotor performance but may have a negative effect on memory. Grant and Chamberlain (2018) found that high caffeine use increased impulsive behaviors and led to poor decision making and risky behavior. However, it is important to note that most research is on extremely high doses of caffeine, and not on the average caffeine or coffee consumer.

The physiological effects of caffeine include high blood pressure, high heart rate, and an increased basal metabolic rate. Childs and de Wit (2006) found an increase in blood pressure and a physical activation of the body when participants consumed caffeine. Childs and de Wit (2006) and Hart and Ksir (2022) found an increase in heart rate, as well as an irregular heart rate in caffeine users. Hart and Ksir (2022) found an increase in the basal metabolic rate of chronic caffeine users. Thölke et al. (2025) found that caffeine disrupts individuals’ sleep patterns, making it harder to fall asleep and reducing the total sleep time and quality. Wang, Guo, and Chen (2025) found that caffeine impacts the composition of our gut microbes, which can then suppress appetite and metabolic regulation.

There is strong evidence that people will build a tolerance to caffeine when regularly consuming it. Lara et al. (2019) found that after 2–3 weeks of regular use, the benefits of caffeine significantly decreased. Meredith et al. (2013) found that there were significant physical and psychological dependence symptoms, which could be seen as withdrawal symptoms. The physical symptoms they found were headaches, fatigue, and decreased alertness (Meredith et al., 2013). The psychological symptoms were continued use despite the negative effects, continued use despite trying to quit, and a craving and reliance for mood or performance, as well as anxiety, irritability, and disruptions in sleep (Meredith et al., 2013). These symptoms begin to occur within 12–24 hours, and peak symptoms tend to occur within 20–48 hours (Meredith et al., 2013). The main reason these symptoms occur is because of the brain’s alteration of the adenosine receptors. Caffeine blocks adenosine, and as a result the brain increases adenosine receptors (Meredith et al., 2013). When the caffeine is gone, the adenosine receptors become overactive, producing withdrawal symptoms (Meredith et al., 2013).

Although caffeine is considered safe when consumed in moderation, excessive intake can cause several short-term side effects, including anxiety, nervousness, jitteriness, insomnia, an increased heart rate, and stomach discomfort. Long-term heavy use may contribute to chronic sleep disturbances, elevated blood pressure, and heart rhythm abnormalities in some individuals. While caffeine overdose is uncommon, extremely high doses can result in seizures, irregular heart rhythms, and, in severe cases, respiratory failure. There are many long-term and short-term effects of caffeine, as well as risks. Similar to what has been discussed previously, the short-term side effects of caffeine are increased blood pressure, elevated heart rate, nausea, muscle aches, cold or hot spells, jitteriness, blurred vision, and headaches (Harrell & Juliano, 2009; Hart & Ksir, 2022). The long-term health risks are an increased risk for pancreatic cancer, potential reproductive and birth defects, heart disease, and caffeinism (Hart & Ksir, 2022). Caffeine overdose is incredibly rare, but when it does occur it is known as caffeinism. This is caused by an excessive amount of caffeine consumed, which leads to convulsions and respiratory arrest, which is what often causes the overdose and death (Hart & Ksir, 2022). Belayneh and Molla (2020) researched the interaction between caffeine and drugs like antidepressants, antipsychotics, cardiovascular medication, and antibiotics. They concluded that the effects vary depending on the drug, but it can lead to treatment failure, unexpected side effects, toxicity, and an alteration in the drug’s absorption, metabolism, and elimination (Belayneh & Molla, 2020).

Caffeine can be used in different treatment settings, such as for neonatal care. According to Yang et al. (2021), caffeine can be used to treat apnea of prematurity for neonates. Apnea of prematurity is caused by an immature central nervous system, and caffeine can help to block the adenosine receptors, which influences dopamine and serotonin (Yang et al., 2021). When this happens, breathing and oxygen delivery improves (Yang et al., 2021). One off-label medical use for caffeine is a high dose of caffeine for weight loss. Viana et al. (2018) found that caffeine increases resting expenditure and enhances lipid metabolism. As a result, caffeine has been included in some weight loss drugs, but the dosing can be inconsistent (Viana et al., 2018). It is commonly included in medications used to treat migraines and tension headaches because it improves the effectiveness of some pain relievers by constricting blood vessels in the brain. Research has also found that moderate caffeine consumption, generally equal to two or three cups of coffee per day for most healthy adults, may provide health benefits, including improved alertness and a lower risk of developing type 2 diabetes.

To reduce potential risks, individuals should limit excessive caffeine intake, avoid consuming large amounts of energy drinks or caffeine tablets, and avoid combining caffeine with alcohol or other stimulants. Pregnant women and individuals with heart disease, anxiety disorders, or other medical conditions should consult a healthcare provider to determine an appropriate daily intake. Because caffeine is legal, inexpensive, and widely available, many people assume it is completely harmless. While moderate consumption is generally considered safe for most healthy adults, research has shown that excessive use can contribute to dependence, withdrawal, anxiety, sleep disturbances, and other health concerns.

Caffeine, while widely studied, still has some myths and controversies. For instance, people believe caffeine gives you energy. However, caffeine does not give you energy; it just blocks the adenosine receptors, making you feel more alert and awake. Another myth about caffeine is that if you drink it early, you will have no problem going to sleep. The half-life of caffeine is about 3 hours, so it stays in the system longer than you would think (Hart & Ksir, 2022). These two myths go along with the media’s portrayal of caffeine. Caffeine does improve alertness and reduce fatigue, but it takes time to get into your system and does not stay forever (Hart & Ksir, 2022). One stigma that can be seen frequently is the idea of the pressures of a productivity culture. Today’s culture has drawn a fine line between the use and dependence of caffeine, which can be seen in this stigma. One of the most debated scientific topics of caffeine is dependence vs. addiction. We have seen that people build a tolerance to caffeine, but scientists debate whether caffeine dependence should be seen as an addiction. Caffeine affects the same reward pathways that drugs like nicotine or opioids do, yet it is not considered an addiction (Hsu, Wang, & Chiu, 2010). So, while there is still a debate, caffeine remains a dependence and not an addiction.

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