Outlawed for Race, Not Health
In 1875, opium became the first drug to be outlawed in the United States. William White wrote in his 1979 essay, "There is little doubt that this law was aimed specifically at the Chinese and reflected more an attempt to control the Chinese as an economic group than it did a concern about the drug opium." (This was far from the last time racism would rear its ugly head in the realm of drug legislation and enforcement.)
At the turn of the 20th century, government in the US became more concerned with various substances "corrupting" the populace, and a few narrow-minded people lobbied to make certain substances illegal. Most of the outlaw hearings before Congress were brief and had little supporting evidence of actual harm done to users or others; in fact, the debate on outlawing marijuana in 1937 lasted less than two minutes. (More)
Attempts to Outlaw Alcohol, Caffeine, and Nicotine
Many drugs that are legal today were attacked in the same way as marijuana and opium. Nicotine (cigarettes) were illegal in a number of states in the first half of the 20th century; caffeine (coffee) was attacked as well. Over 40 brands of soda contained cocaine, and switched to the more addictive drug caffeine after cocaine was made illegal. Cocaine itself was outlawed due to fear of what the media appallingly called "Cocainized Niggers". Despite any supporting evidence, those pushing for prohibition feared that black men would use cocaine and go on a killing rampage. (More)
1919 brought the prohibition of alcohol, possibly Western culture's favorite drug. Dangerous, habit-forming, physically harmful, and with the tendency to produce violent behavior in its users, this drug is truly one of the worst. But it was simply too far entrenched into American culture, and it was made legal again barely more than a decade later.
Nature is Illegal
One shocking thing about many of these laws is that they make nature itself illegal. Poppy flowers (the source for opium), marijuana plants, and peyote cacti (a source of mescaline) all grow naturally and abundantly in the United States. But the government hasn't just outlawed consumption of these plants; it's actually outlawed the plants themselves. Poppies, which are a particular favorite because of their beauty, are often ripped out of private gardens by over-zealous DEA agents. In most cases the growers had no idea the flower can be turned into a drug.
People continued to discover new psychoactive substances, while the government continued to outlaw them - but only if it could be used for pleasure. Recreational drug use becomes a symbol of hedonism, which lawmakers connect directly to the downfall of organized society. As a result, many useful medicines are outlawed. Heroin, an excellent painkiller, is made off-limits to hospitals, who are forced to use the less pleasurable (but also less effective) derivative opiates such as morphine. In fact, the government found that the flood of chemicals people were discovering for psychoactive use was impossible to keep up with. Many states passed substance analogue laws, which allows them to prosecute for possession or consumption of a substance which is chemically similar to one which is already illegal. In effect, they've outlawed a virtually infinite number of drugs before they've even been discovered.
This is beyond just draconian; it's harmful. It's possible, and likely, that there exists a drug which could become a treatment or cure for cancer, AIDS, or other currently incurable diseases, but it's already illegal due to analogue laws. Researchers don't even have the option to examine it and find out. Being a cousin to a drug that could potentially be used for pleasure seems to be good enough reason to make it off-limits for reseach.