More than 20 million girls and women in the US abuse drugs and alcohol and 30 million more are addicted to cigarettes.  Women, pound for pound, not only get more drunk or higher faster than men, but also become addicted more easily.  Teenage girls now smoke, drink and abuse drugs as often as teenage boys.  For certain drugs, such as prescription painkillers, the abuse rate is higher in girls than boys.  Yet even as the rate of abuse becomes more equal, psychological and physiological factors combine to ensure that females are more greatly affected by drugs and alcohol.
    Each single drink hits a woman like a double.  A woman's body contains less water and more fatty tissue which increases alcohol absorption- compared to a male body.  And women have a lower activity level of an enzyme called alcohol dehydrogenase, whcih breaks down alcohol.
    The risk of addiction to alcohol and drugs, including nicotine, is approximatly doubled as well.  The reason may be hormonal of psychological.  Males and females abuse drugs for different reasons.  Teenage girls are more likely than boys to abuse illegal substances in order to loose weight, relieve stress or boredom, improve their mood, reduce sexual inhibitions, self-medicate depression, and increase confidence.

There may be some exceptions, but you're right.  Girls generally have a lower tolerance than men.  This should be clearly understood to all girls however.  It's not smart to go shot for shot with any guy.