Prohibition

Prohibition is not compatible with a free society. Prohibiting acts of violence is one thing, but laws that prohibit the use of certain substances—food, drugs, or alcohol—by adults is a dangerous intrusion on personal liberty. Prohibition is motivated by busybodies who have a gross misunderstanding of the unintended consequences of attempts to improve other people's habits and character through government force. Time and time again, we are shown that it simply does not work. If there are to be any regulations on the use of certain substances in the United States, it was intended that this should be done by the individual states, not by the federal government.

Ron Paul provides some history on alcohol prohibition, noting the crux of the problem with respect to liberty:

Those who promoted prohibition showed no concern for the large majority who drank alcohol responsibly for enjoyment and whose rights would be violated by prohibition.

To stop the excesses of a few, the many were required to give up their freedom of the choice to enjoy a glass of wine or beer.

And prohibition's impracticality:

Prohibiting any desired substance inevitably leads to a black market, as history has shown countless times, and never achieves its goal of eliminating the use.

As well as a rarity that only the Champion of the Constitution would point out:

The only good thing about the process was that, in 1919, the American people and the U.S. Congress had enough respect and understanding of the Constitution to know that an amendment was required to authorize prohibition.

Modern drug prohibition has led to the incalculably destructive "war on drugs", which has impoverished many nonviolent people, ravaged border cities, and provided the only thing both Christian conservatives and drug dealers can agree upon: its continuation.

Most Americans fail to recognize that throughout most of our history drug laws didn't exist. There's reliable evidence that the laws have done nothing to decrease drug usage while contributing significantly to street crime.
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Government should not compel or prohibit any personal activity when that activity poses danger to that individual alone. Drinking and smoking marijuana is one thing, but driving recklessly under the influence is quite another. When an individual threatens the lives of others, there is a role for government to restrain that violence.

Ron Paul also warns that the principle lost (freedom to put into our bodies what we wish) leads to a government that thinks it knows best what everyone can and cannot consume, and will use force to make sure of it.