When does a stimulating drink become dangerous, even fatal? That's what the US Food and Drug Administration is trying to find out as it investigates a possible link between so-called energy drinks and at least 18 deaths in the United States in recent years. Energy drinks contain a number of stimulants that claim to increase one's attention span and stamina, and even improve sporting performance. But there is growing concern about their harmful effects even as their popularity rises worldwide. The FDA's investigation is focused, in particular, on two energy drinks: 5-Hour Energy, which may be linked to the deaths of 13 people in the past four years; and Monster, which may have contributed to five other deaths. Global energy drink consumption rose 14 per cent year on year in 2011 to 4.8 billion litres - more than 1.5 billion litres higher than in 2007, according to a report from leading food and drink consultancy Zenith International, released in February. Consumption has grown by an average of 10 per cent a year over the past five years. China ranked third for consumption among the 57 countries covered by the report. North America is still the leading consumer, with 36 per cent of global volume in 2011, followed by the Asia-Pacific region at 22 per cent. By 2016, Zenith estimates, global market consumption volumes will have risen 35 per cent to 6.5 billion litres. This rise in consumption of drinks that may not be safe is a worrying trend. In recent years, the Hong Kong Centre for Food Safety notes, several countries have reported possible links between the consumption of energy drinks and cases of cardiac dysrhythmia, seizures, kidney failure and fatalities. Such incidents generally involved improper intake of the beverages, such as drinking them with alcohol, or in greater quantities than recommended. Energy drinks are meant to enhance energy by stimulating the nervous system with ingredients such as caffeine, a selection of B vitamins, taurine (an amino acid) and glucuronolactone (a carbohydrate). Caffeine and taurine have been singled out in research studies as the potential culprits. Though ingredients such as caffeine are not inherently dangerous, problems could arise when these substances are taken in excessive amounts, in combination with other substances, or by the wrong people. "Both caffeine and taurine have been shown to have a direct impact on cardiac function," says Dr James Kalus from the Henry Ford Hospital in Detroit. Kalus was the lead author of a 2009 study which found that healthy adults who drank two cans a day of a popular energy drink experienced an insignificant increase in their blood pressure and heart rate. This could prove harmful to people with a heart condition. Kalus notes that energy drinks should not be confused with sports drinks, which aim to replenish the carbohydrates and electrolytes that a body needs. "If you have high blood pressure, heart problems, or high blood-sugar levels, you shouldn't drink [energy drinks] because they all have high doses of stimulants and sugar," says Mimi Li Shuk-ping, a Hong Kong-registered dietitian. "The caffeine will increase your heart rate and blood pressure, and, in the end, may increase the chance of having a heart attack." The caffeine content of energy drinks varies over a 10-fold range, with some containing the equivalent of 14 cans of Coca-Cola, according to scientists at Johns Hopkins University, who have spent decades researching the effects of caffeine. A regular 330 millilitre can of cola has about 35 milligrams of caffeine, and a 180ml cup of brewed coffee has 80mg to 150mg. Energy drinks can contain from 50mg to more than 500mg of caffeine. In an article published in 2008 in the journal Drug and Alcohol Dependence , the scientists highlight the potential health risks of excessive caffeine - and the need for warning labels on energy drink cans. Caffeine intoxication, a clinical syndrome included in the World Health Organisation's International Classification of Diseases, is marked by nervousness, restlessness, anxiety, insomnia, gastrointestinal upset, tremors, rapid heartbeat (tachycardia), psychomotor agitation (restlessness and pacing) and, rarely, death. By contrast, little is known about the effects of heavy or long-term taurine use. Taurine, a non-essential amino acid derivative often found in meat and fish, plays a range of roles in musculoskeletal and nervous-system development. Dietitian Katherine Zeratsky, writing on the Mayo Clinic website, notes: "Some studies suggest that taurine supplementation may improve athletic performance, which may explain why taurine is used in many energy drinks. Other studies suggest that taurine combined with caffeine improves mental performance, although this remains controversial." Up to 3,000mg a day of supplemental taurine is considered safe, says Zeratsky. Any excess is excreted by the kidneys. A single Red Bull contains 1,000mg of taurine. Guarana is another common ingredient in energy drinks. It is a plant found in the Amazon, the seeds of which contain about twice the caffeine level of a coffee bean. The possible adverse effects of these stimulants are compounded when energy drinks are mixed with alcohol, such as in Red Bull vodkas, or Jagerbombs. A study published last year in the journal Alcoholism: Clinical & Experimental Research found that an energy drink alters the reaction to alcohol a drinker experiences when compared to drinking alcohol alone. The latter leads to impulsive behaviour, but energy drinks provide an enhanced feeling of stimulation, leading to possibly risky scenarios. "Energy drinks have become enmeshed in the subculture of partying," researchers said in a commentary published last year in the Journal of the American Medical Association . "The practice of mixing energy drinks with alcohol has been linked consistently to drinking high volumes of alcohol per drinking session and subsequent serious alcohol-related consequences such as sexual assault and driving while intoxicated," the journal said. There's another common ingredient present in high doses in energy drinks that has proven time and again to be disastrous for health: sugar. Energy drinks, for the most part, have as much, or more, sugar than soft drinks, and excessive sugar has been irrefutably linked to chronic diseases such as obesity, heart disease and cancer. These drinks are also bad for your teeth. A study published recently in General Dentistry , the clinical journal of the Academy of General Dentistry, found that the high acidity levels in energy drinks caused twice as much damage as sports drinks to tooth enamel, the glossy outer layer of the tooth. The recent incidents in the US have led to renewed concern over energy drinks, and rising calls for tighter regulation of the beverages. In the US, energy drinks fall in the category of dietary supplements, meaning they are exempt from FDA approval and rules requiring nutritional labelling information. In Europe, drinks with more than 150mg of caffeine per litre have to be labelled "high caffeine content". France had imposed a ban on Red Bull for 12 years before backing down in 2008 after European regulators said they found no evidence of danger. There is no specific legislation governing the standard of energy drinks in Hong Kong. With the recent incidents in the US, perhaps it is time to review the regulations of such drinks here. Even if that energy drink probably won't kill you, there are healthier ways to get a boost. "Have some coffee, go outside and take a few minutes' walk to get some fresh air and increase your oxygen level. Take a shower or a 20-minute nap," says Li. "All of these methods are preferable to energy drinks."