RISKS

Guarana holds a GRAS-status in the U.S. which means that is is generally recognized as safe.  However, you should consult your physician before taking Guarana supplements, especially if you are taking drugs for any medical condition. Additionally, you may want to seek the advice of a practitioner trained in the uses of herbal and/or health supplements.

Obviously, Guarana should not be used by people who are sensitive to caffeine or xanthines, but there are other concerns as well.

People with heart conditions, diabetes, high blood pressure, epilepsy, overactive thyroid, anxiety, insomnia, and kidney disease should only use Guarana under the supervision of their doctor.

The safety of Guarana in pregnant or nursing women has not been established. Since many doctors recommend limiting caffeine during pregnancy and nursing, Guarana should be avoided because it isn't possible to know how much caffeine you may be consuming with Guarana products.

OF NOTE:

A report published in the Journal of Herbal Pharmacotherapy described the case of a heart rhythm abnormality called premature ventricular contraction associated with two herbal supplements that both contained large doses of Guarana.

Cardiovascular adverse reactions associated with Guarana: is there a causal effect?
J Herb Pharmacother. 2002

From anecdotal evidence of excessive consumption of energy drinks, Guarana may contribute (alone or in combination with caffeine and taurine) to onset of seizures.

New-onset seizures in adults: possible association with consumption of popular energy drinks
Epilepsy Behav. 2007 May

Guarana Overdose
Initial symptoms of Guarana overdose include difficulty urinating, vomiting, and abdominal cramps and spasms. If you suspect a Guarana overdose, seek medical attention immediately.

DRUG INTERACTIONS

Guarana should not be taken with any products containing ephedra. Serious adverse effects have been reported with this combination. It may increase the risk of stroke, hemorrhage, myocardial infarction, and sudden death and has been associated with increases in heart rate, blood pressure, and potentially harmful changes in glucose and potassium levels.

Short-term metabolic and hemodynamic effects of ephedra and Guarana combinations
Clin Pharmacol Ther. 2005 Jun

Acute hemorrhagic myocardial necrosis and sudden death of rats exposed to a combination of ephedrine and caffeine
Toxicological Sciences (2005)

Guarana has been found to decrease platelet aggregation and thromboxane synthesis, so it may increase the risk of bleeding when taken with aspirin, anticoagulants such as Warfarin (Coumadin®), and platelet inhibitors such as Ticlopidine (Ticlid®), Clopidogrel (Plavix®).

A novel property of an aqueous Guarana extract (Paullinia cupana): inhibition of platelet aggregation in vitro and in vivo
Brazilian Journal of Medical and Biological Research (1988)

All drug interactions that can occur with caffeine would be expected to occur with Guarana as well.  For that reason, Guarana should not combined with MAO-inhibitors or stimulant drugs such as Ritalin.