The Supreme Court of Canada struck a blow against the so-called "two-beer" defence, in which defendants accused of drinking and driving claim that they did not consume enough to be drunk, despite failing a breathalyzer test.

In the 7-2 split decision, the court decided to uphold the breathalyzer results in the separate cases of a Nova Scotia man and an Alberta man.

In each case, the men used toxicology experts to estimate the amount of the alcohol in their system, saying that they actually "straddled" the 80 mg limit on blood alcohol levels.

It is a common defence tactic to have experts testify that because of the accused's personal alcohol absorption rate, combined with other variables, such as length of drinking time or amount of food consumed, that the person was not as drunk as the breathalyzer results suggested.

But the court's ruling says that there are too many variables involved in how the body metabolizes alcohol and that the breathalyzer's result should be considered the legal standard.

"Because it is scientifically undisputed that absorption and elimination rates can vary from time to time, nothing is really gained by post-offence testing of an accused's elimination rate," Justice Louise Charron said on the behalf of the majority judges in the ruling.

"The breathalyzer test provides legal proof that the accused 'consumed alcohol in such a quantity' that it put him or her over 80 mg contrary to s. 253 of the Criminal Code."

The ruling said that to admit the 'two-beer offence' - so-called because of the common defendant claim that they could not be drunk because they only had two beers - would "obviously fly in the face" of government efforts to control drinking and driving.

"Straddle evidence merely confirms that the accused falls into the category of drivers targeted by Parliament -- namely, those who drive having consumed enough alcohol to reach a blood alcohol concentration exceeding 80 mg," the decision read.