San Francisco, Calif. — the epicenter of the 1960s' psychedelic revolution — is notorious for drugs. But in the new millennium, the city, especially the gay community, is struggling to recover from a meth epidemic. The enabler of this situation is the Asian cartel, which has been poisoning San Francisco with high quality meth for almost 25 years — but the Mexican cartels are intent on taking over. Nat Geo goes inside one of the worst drug ghettos in America — the heart of the drug trade.

Kingston is in the hands of highly organized and warring drug gangs. They manage drug trafficking and distribution at all levels. With the patronage of politicians, they fight hard to protect their turf in a city where poverty and guns drive the drug trade.

Chicago, the biggest open-air crack and heroin market in America is at saturation point. The result: record levels of overdoses and homicides, as gangs fight over drug turf.

Houston's drug hub is the 'Bloody Nickel' - the Fifth Ward. Five square miles of 24/7 drug and party action. Gangs work across the ethnic divide to keep the drugs flowing; while cops and Cartels vie with each other for control.

Denver - the mile high city - has just legalized recreational use of Marijuana. The city's gang-banger dealers have battled falling profits since medical Marijuana became law, and forced them to push new products. This latest move is last straw.

Miami - once America's cocaine capital; but no more. The War on Drugs has hit home. Today dealers and users play cat and mouse with the cops and live in dread of the Feds. And a new look drug scene has gone underground.

In Kensington north Philadelphia, heroin, crack cocaine and PCP are dealt from a hundred different corners. Kensington is a giant drug drive thru. But for the addicts and dealers it's a deadly trap that threatens to drive them literally insane.

Phoenix, Ariz., is the wholesale drug capital of America, under the control of one of Mexico's most powerful and ruthless drug organizations -Â- the Sinaloa Cartel. Almost 200 miles north of the Mexican border, the city has become the major stopping off point for the large-scale distribution and transshipment of marijuana, heroin and methamphetamine up from Mexico and out to the rest of the country.

Music and drugs go hand in hand in Nashville. For musicians who don't make it, drugs are the easiest way to make a living. National Geographic talks to a user and freelance cook who makes methamphetamine at home to cover his costs and get high. Then, learn how the 19th Judicial District Drug Task Force is faced with cleaning up after meth cooks' Â- operations that are both extremely dangerous and prohibitively expensive.

See the underground world of drugs through the eyes of its makers. A former British gangster serves as guide into this illicit underworld, visiting a secret hash-making location nestled in the mountains. Check out growers in Holland who are leading a horticultural revolution in high-grade cannabis. Follow a batch of raw opium from an Afghan poppy field to an Iranian heroin lab. Follow the supply chain of cocaine through the eyes of Colombian peasant farmers producing the drug in a paste.

The sale of drugs accounts for billions of dollars in illegal commerce across the globe, bringing untold wealth to a select few. But these men and women who live lavish lifestyles off the sale of drugs rarely associate themselves with the ugly underbelly where their products hit the streets — with users. These stories show that wherever people use — be it an island paradise or a cold city street — they can get lost in the shuffle. This episode will go deep into the stories of the users.

Every pill popped, pipe smoked and shot injected comes with a price tag. Some dealers focus their efforts on high society users — paying top dollar for the best quality stuff, while others focus on quantity — selling users just enough to scratch the itch and come back for more. Either way, the smuggling and selling of drugs brings in big money. And some of the people handling it are the best in the business.