As NATO and the Afghan government put more pressure on poppy production (which produces opium, which if often refined into heroin), many farmers, and drug gangs, are switching to marijuana (cannabis). Afghanistan has become the world's largest producer of hashish (a concentrated form of marijuana that is easier to export). Most of the marijuana is grown where poppies are also grown, but more marijuana is found elsewhere. About half the provinces have significant marijuana production, while only one of the 34 provinces account for most of the heroin.
The most widely used drug in the world is actually marijuana (and it's refined version, hashish). There are about 170 million users of these products worldwide, compared to 30 million for cocaine and heroin. Many users live in rural areas where marijuana grows wild and legal restrictions are not energetically enforced. But in many urban areas, marijuana is a major source of income for gangsters, and some terrorist groups. Not as profitable as cocaine and heroin, and harder to smuggle (because of the bulk), it is still a major threat because it has such a large market.
Marijuana has been used by humans for over 5,000 years. It's been used for medicinal purposes, to enhance food recipes or religious ceremonies and, well, to just get high. Because cannabis is not as powerful as opium (another ancient drug, but more expensive to produce), and a lot easier to get (it grows wild in many parts of the world, especially Central Asia) societies, and governments, tended to ignore its impact. Alcoholic beverages were seen as more of a public menace.
Hashish, however, can be potent stuff. Especially if it is extracted from strains of cannabis bred to have higher amounts of THC (the major active ingredient that gives you the buzz). This breeding effort has more than tripled the THC content of commercial cannabis strains over the last three decades. For Afghan farmers, cannabis is cheaper and easier to grow than poppies, and less likely to attract the attention of NATO or Afghan drug control forces. Cannabis is not as profitable as poppies, but it's still more profitable than wheat. Who really gets hurt by this switch are the drug gangs, which make a lot less money on hashish, compared to heroin. That means less money for their Taliban allies as well.
If the government is successful in building more roads, and protecting truck traffic, more farmers can produce more lucrative crops (like saffron, which sells for much more than poppies, and is legal).