Similarly, nicotine may speed the metabolism of caffeine. Because caffeine is a competitive antagonist for adenosine 1 & 2a primarily at striatal sites, it may also selectively increase the efficacy of D2 receptors, given evidence that D2 depleted mice show reduced effects of caffeination.
http://scienceblogs.com/developingintelligence/2008/02/11/optimally-wired-a-caffeine-use/
It has re-confirmed that for the average adult, moderate daily caffeine intake at dose levels of 400 mg/day is not associated with any adverse effects. Data has shown, however, that women of childbearing age and children may be at greater risk from caffeine.
http://hc-sc.gc.ca/fn-an/securit/addit/caf/food-caf-aliments-eng.php
It will be important for future work to examine caffeine’s effect on tasks thought to require NoGo pathway activity. A variety of other chemicals may work on the A2 receptor in similar (SCH 58261, ZM 241385, CSC, KF17837) or opposing ways (CGS 21680, APEC, 2HE-NECA).
http://scienceblogs.com/developingintelligence/2008/02/11/optimally-wired-a-caffeine-use/
Nonetheless, Health Canada suggests that daily caffeine intake for this age group be no more than 2.5 mg/kg body weight. This is because the maximum adult caffeine dose may not be appropriate for light weight adolescents or for younger adolescents who are still growing. The daily dose of 2.5 mg/kg body weight would not cause adverse health effects in the majority of adolescent caffeine consumers. This is a conservative suggestion since older and heavier weight adolescents may be able to consume adult doses of caffeine without suffering adverse effects.
http://hc-sc.gc.ca/fn-an/securit/addit/caf/food-caf-aliments-eng.php
Between 20-200mg per hour may be an optimal dose for cognitive function. Caffeine crosses the blood-brain barrier quickly (owing to its lipid solubility) although it can take up to 45 minutes for full ingestion through the gastro-intestinal tract.
http://scienceblogs.com/developingintelligence/2008/02/11/optimally-wired-a-caffeine-use/
Once you start drinking four or more cups of coffee, or otherwise consuming 500 mg to 600 mg of caffeine daily, you are more likely to experience the ill effects of caffeine. Coffee is one of the most abundant sources of caffeine, although other consumable items contain the drug.
http://www.livestrong.com/article/471513-the-maximum-daily-dosage-of-caffeine/
Some evidence indicates high heritability for caffeine addition (note: this is based on interviews of twins) and others are advocating the recognition of caffeine addiction as a bona fide mental disorder. Withdrawal symptoms can onset within 12 to 24 hours of caffeine consumption and last between 2 and 9 days. There are more cognitive concerns here as well. For one, caffeine probably follows the Yerkes-Dodson law, in which a moderate dose is superior to too little or too much.
http://scienceblogs.com/developingintelligence/2008/02/11/optimally-wired-a-caffeine-use/
The recommended maximum caffeine intake levels shown in the table below are based on the most current research available. Health Canada has not developed definitive advice for adolescents 13 and older because of insufficient data.
http://hc-sc.gc.ca/fn-an/securit/addit/caf/food-caf-aliments-eng.php
Unlike vitamins, minerals and other nutrients, there is no set limit for a maximum daily dose of caffeine. Depending on your own sensitivity to caffeine, you could max out with a single cup of coffee, which generally contains up to 100 mg of the stimulating drug. The maximum limit is any amount that changes caffeine's pleasant side effects into unpleasant ones. Most adults can consume 200 mg to 300 mg of caffeine per day, which translates to two to four cups of coffee, without any health concerns or negative side effects.
http://www.livestrong.com/article/471513-the-maximum-daily-dosage-of-caffeine/
Caffeine (either added to a weight-loss supplement or as an herbal source that naturally contains caffeine, such as guarana (Paullinia cupana), kola (or cola) nut (Cola nitida), and yerba mate (Ilex paraguariensis), is commonly found in dietary supplements promoted for weight loss.
https://ods.od.nih.gov/factsheets/WeightLoss-HealthProfessional/
caffeine maximum amount based on weight
The maximum amount of caffeine is 2.5 milligram per kilogram based on body weight. The maximum amount of caffeine is 2.5 mg per kg body weight.