Until recently there wasn't much talk about drugs in Japanese culture, because there weren't any drugs in Japan to talk about. Recently, however, there's been a spat of news stories involving Japanese college students getting caught with marijuana. Now this would be laughable as a news story in London or Los Angeles, but in Japan it's a worthy news subject.
In Japan, unlike most of the west, there are no distinctions made between heroin and marijuana, for example. In Japan, both of these drugs are illegal and therefore taboo. In many ways this is a far saner attitude than in America where marijuana is illegal, but is seen as benign by most of the culture and some in government - and so people ignore the law.
I can only speculate why pot is now making an appearance in Japanese culture. My guess is that Japanese students who go abroad to Australia, America and Europe are exposed to it and pick up the message that it is a "cool" and harmless thing to do.
Being an island nation, Japan has historically been able to control much of what comes in an out of Japan. We see this when it comes to immigration, narcotics and even guns. This is not to ignore the fact that the Japanese mafia (yakuza) has been dealing with drugs and guns for years, but most Japanese aren't touched by the underworld.
Japanese culture generally condones sloppiness and excess when it comes to alcohol use. Even the most mild-mannered Japanese businessman (salaryman) is allowed to get crazy and make a fool of himself when he is drinking. I can only image the absolute tawdriness of Japanese people who are under the influence of illegal drugs.
I wonder if drug use in Japaneses society will produce a generational split along the lines of the 1960s in America, when the college kids of that generation decided that illegal drug use was a private affair and no state or Church was going to tell them anything different.