Guarana may have an adverse effect on sleeping disorders such as insomnia and can also result in heart palpitations or anxiety. — Gruenwald J, Brendler T, Jaenicke C — PDR for Herbal Medicines. 1998, 1017–8.

The authors review the case of a thirty year old patient who used products containing Guarana and an occasionally used non-steroid painkiller. The history and laboratory results showed acute renal and hepatic failure.
– Orv Hetil. 2007 Mar 4;148(9):421-4. — Acute renal failure caused by plant extract. — Vágási K, Degrell P, Késoi I, Kovács T, Pintér I, Molnár B, Cseh J, Nagy J, Wittmann I. Pécsi Tudományegyetem, Altalános Orvostudományi Kar II.

Do not use if you are pregnant, breastfeeding or taking other medications.

Guarana supplements, can result in an increase in heart rate and blood pressure and dangerous actions on glucose and potassium homeostasis. These effects can be detrimental in persons with hypertension, atherosclerosis, or glucose intolerance, conditions that are strongly associated with obesity.
– Clin Pharmacol Ther. 2005 Jun;77(6):560-71. — Short-term metabolic and hemodynamic effects of ephedra and guarana combinations.Haller CA, Jacob P, Benowitz NL.

Guarana contains high levels of caffeine and should not be used with similar herbs or products that stimulate the heart or blood pressure.
– Ann Pharmacother. 2005 Mar;39(3):574-5. Epub 2005 Jan 18. — Potential for toxicity with use of bitter orange extract and guarana for weight loss.Hess AM, Sullivan DL.

The ease of accessibility to herbal products could result in greater potential for recreational abuse and subsequent complications presenting to emergency departments. Health care professionals must be cognizant of this emerging problem as increased media coverage and marketing have made these products accessible and recognizable to many young adults and teenagers.
– Emerg Med Clin North Am. 2007 May;25(2):435-57; — Herbal drugs of abuse: an emerging problem.Richardson WH 3rd, Slone CM, Michels JE.

The strongest concerns in the literature appear for ephedra, bitter orange, and guarana. An audit revealed numerous weight-loss products available to consumers, yet there is little evidence to support the effectiveness of the ingredients identified and many potential adverse reactions.
– J Am Diet Assoc. 2006 Dec;106(12):2045-51. — Availability of weight-loss supplements: Results of an audit of retail outlets in a southeastern city.Sharpe PA, Granner ML, Conway JM, Ainsworth BE, Dobre M.

Guarana (Paullinia cupana) is a popular herb native to the Amazon Basin and used extensively in soft drinks in Brazil, other Latin American countries, and more recently in the United States. Extracts derived from the dried seeds of guarana possess strong anti-platelet aggregatory properties. In this study, an active fraction containing this activity was purified and analyzed by high-performance liquid chromatography/mass spectrometry (HPLC/MS) techniques. It was noted that this fraction contains catechins, epicatechins, and their dimers, with a small amount of caffeine. It is suggested that complexes containing caffeine and catechins (and their dimers) might be responsible for anti-platelet aggregatory activity in guarana seeds and might offer health benefits towards decreasing risk of thrombosis and cardiovascular disease.

– Studies on the nature of anti-platelet aggregatory factors in the seeds of the Amazonian Herb Guarana (Paullinia cupana). — Ravi Subbiah MT, Yunker R. — Int J Vitam Nutr Res. 2008 Mar;78(2):96-101.

Guarana (Paullinia cupana) extracts are most commonly used in Western markets as putatively psychoactive food and drink additives. This double-blind, randomised, placebo-controlled, parallel groups study assessed the acute effects of either a vitamin/mineral/guaraná supplement or placebo drink in 129 healthy young adults (18-24 years). This research supports previous findings demonstrating guarana’s cognition enhancing properties and provides evidence that its addition to a multi-vitamin-mineral supplement can improve cognitive performance and reduce the mental fatigue associated with sustained mental effort.
– Appetite. 2008 Mar-May;50(2-3):506-13. Epub 2007 Oct 30. — Improved cognitive performance and mental fatigue following a multi-vitamin and mineral supplement with added guaraná (Paullinia cupana). — Kennedy DO, Haskell CF, Robertson B, Reay J, Brewster-Maund C, Luedemann J, Maggini S, Ruf M, Zangara A, Scholey AB.