Energy drink manufacturers do not accurately list the levels of caffeine in their drinks, a new report says.

Consumer Reports magazine measured caffeine levels in 27 energy drinks and shots, testing three lots of each product. The amount of caffeine found in the drinks ranged from 6 milligrams to 242 milligrams per serving, with some packages including more than one serving.

While caffeine can boost energy, it can also affect heart rhythms and increase blood pressure. Research suggests 400 milligrams is the maximum daily limit of caffeine for healthy adults. That figure is 200 milligrams for pregnant women, and 45 to 85 milligrams for children.

The drink 5-hour Energy Extra Strength had the highest amount of caffeine, while 5-hour Energy Decaf had the lowest, the study found.

The magazine also found that in five of the 16 drinks that reported caffeine levels, namely Arizona Energy, Clif Shot Turbo Energy Gel, Nestlé Jamba, Sambazon Organic Amazon Energy, and Venom Energy, the actual amount of caffeine was much more than what was listed on the labels.

“Some of the energy drinks underestimated the amount of caffeine listed on the label by 20 per cent or more,” says Consumer Reports’ deputy health editor Gayle Williams.

Eleven drinks did not list caffeine levels at all, which Consumer Reports suggested might be to protect companies’ proprietary blends.

According to a representative of the Monster Beverage Corporation, the company does not list levels because in the U.S. “there is no legal or commercial business requirement to do so, and also because our products are completely safe, and the actual numbers are not meaningful to most consumers.”

However, last week, the parents of a 14-year-old California girl filed a wrongful death suit against the company after their daughter drank two 24-ounce Monster Energy Drinks in 24 hours. According to an autopsy, the girl died of cardiac arrhythmia due to caffeine toxicity.

A 24-ounce can of Monster Energy Drink contains 240 milligrams of caffeine, and the girls’ parents say the company did not warn about the risks of consuming the drink.

Consumer Reports says Monster Energy Drink is one of 17 products whose labels caution against consumption by children, pregnant or nursing woman, and those who are caffeine-sensitive. In addition, the company, along with eight others, recommends a daily limit.

Monster Energy Drink is also suspected to be linked to five deaths and a non-fatal heart attack in a report being investigated by the U.S. Food and Drug Administration.

Health Canada says it is moving energy drinks from the "natural health product" category to the food category. This means labelling will be mandatory and no drink will be able to have more than 180 mg of caffeine.

With files from The Associated Press and CTV's Seamus O' Regan

How much caffeine is too much? Should the government introduce tougher regulations for energy drinks? Leave your thoughts in the comments below.