Effects of Caffeine
Caffeine is a competitive inhibitor of adenosine. This means that when caffeine runs through the body, caffeine outcompetes adenosine and occupies the active site of the enzyme phosphodiesterase, which would normally degrade adenosine. As a result, this mechanism sheds light not only on the effects of caffeine on the body, but also the effects of the absence of caffeine on the body (e.g. withdrawal).
Positive Effects of Caffeine
Sympathetic nervous system: the sympathetic nervous system is a part of the autonomic nervous system involved with body’s fight-or-flight response. Caffeine prevents the enzymatic degradation of adenosine located in postsynaptic cells. This increases the strength of transmission of neural signals in this system. 
Metabolism: According to Belza et al, as little as 50mg of caffeine significantly increases the resting energy expenditure. This means that without doing anything, one metabolizes more with caffeine. Through the sympathetic nervous system, caffeine promotes fat lipolysis and exercise associated fatty acid oxidation. 
Athletic Performance: After consumption of caffeine, the concentration of serotonin increases in the brainstem regions. This increases spinal motor neurons. As a result, there is an increase in the firing of skeletal muscle motor units, which means better energy utilization and lower levels of exertion. 
Mental/Cognitive Improvement: The cerebral blood flow is proportional to the amount of caffeine consumed . As a result, there is an increase in efficiency with neurotransmissions involved in the cerebral cortex. This results in improvements with alertness, concentration, decision-making, problem solving, and neuromuscular coordination. 
Mood: Studies from Quinlan et al have shown that caffeine consumption of 100mg leads to mood elevation and a decrease in anxiety. 
Negative Effects of Caffeine
Withdrawal: The understanding of how caffeine works in the body is not the direct effects of caffeine. Rather, it’s the responses that caffeine produces due to the reversal of withdrawal effects. Withdrawal effects include headaches, difficulty to concentrate and sleep, and pain in the stomach and joints. Moreover, withdrawal effects have been shown to lead to poorer academic performance, which is contrary to the initial effects of caffeine. 
Pregnancy: For pregnant women who habitually consume caffeine, withdrawal effects have been seen during labor. Caffeine can cross the human placenta, and the caffeine concentration in the fetus can reach as high as the mother’s. Caffeine consumption during pregnancy has been shown to be associated with fetal growth restriction. A study by Bech et al found that the average weight of babies born from mothers who consumed caffeine during pregnancy was 16g less than those who did not consume caffeine . A study by Weng et al demonstrated that daily consumption of caffeine during pregnancy led to an increase in risk of miscarriages .
Gateway: Studies have shown that the caffeine in energy drinks may be a “gateway” into other substances, notably nicotine and alcohol .
Over-caffeination: While caffeine does provide some benefits to humans mentally and physiologically, there is a limit to how much we can consume. For adults, 12.5-100mg/day of caffeine is associated with its positive effects. 4-12mg caffeine per kg of body mass is associated with anxiety, jitteriness, headaches, and fatigue. Less than 400mg of caffeine per day is considered safe. However, acute clinical toxicity may occur after consumption of 1g of caffeine, and 5-10g of caffeine can be lethal. 150-200mg per kg of body mass (roughly 80-100 cups of coffee) is estimated to be median lethal dose .