Alcohol

Alcohol is one of the two gateway drugs. This is because as it's effects start to wear off, addicts want to use another drug to get them high. The reason the U.S. alcohol age is 21 is because your brain is not really done developing from adolecence until your 21.

Alcohols often have an odor described as 'biting' that 'hangs' in the nasal passages. Ethanol in the form of alcoholic beverages has been consumed by humans since pre-historic times, for a variety of hygienic, dietary, medicinal, religious, and recreational reasons. The consumption of large doses results in drunkenness or intoxication (which may lead to a hangover as the effect wears off) and, depending on the dose and regularity of use, can cause acute respiratory failure or death and with chronic use has medical repercussions. Because alcohol impairs judgment, it can often be a catalyst for reckless or irresponsible behavior. The LD50 of ethanol in rats is 10,300 mg/kg.[5]
Other alcohols are substantially more poisonous than ethanol, partly because they take much longer to be metabolized, and often their metabolism produces even more toxic substances. Methanol, or wood alcohol, for instance, is oxidized by alcohol dehydrogenase enzymes in the liver to the poisonous formaldehyde, which can cause blindness or death.[2]
An effective treatment to prevent formaldehyde toxicity after methanol ingestion is to administer ethanol. Alcohol dehydrogenase has a higher affinity for ethanol, thus preventing methanol from binding and acting as a substrate. Any remaining methanol will then have time to be excreted through the kidneys. Remaining formaldehyde will be converted to formic acid and excreted.[6][7]
Methanol itself, while poisonous, has a much weaker sedative effect than ethanol. Some longer-chain alcohols such as n-propanol, isopropanol, n-butanol, t-butanol and 2-methyl-2-butanol do however have stronger sedative effects, but also have higher toxicity than ethanol.[8][9] These longer chain alcohols are found as contaminants in some alcoholic beverages and are known as fusel alcohols,[10][11] and are reputed to cause severe hangovers although it is unclear if the fusel alcohols are actually responsible.[12] Many longer chain alcohols are used in industry as solvents and are occasionally abused by alcoholics,[13][14] leading to a range of adverse health effects.[15]

I found this work on http://en.wikipedia.org/wiki/Alcohol