California will launch the world’s largest regulated commercial market for recreational marijuana today, as dozens of newly licensed stores catering to adults who enjoy the drug for its psychoactive effects open for business up and down the state.

It becomes the sixth U.S. state, and by far the most populous, venturing beyond legalized medical marijuana to permit the sale of cannabis products of all types to customers at least 21 years old. Colorado, Washington, Oregon, Alaska and Nevada were the first to introduce recreational pot sales on a state-regulated, licensed and taxed basis. Massachusetts and Maine are on track to follow suit later this year.

With California and its 39.5 million residents officially joining the pack, more than one in five Americans now live in states where recreational marijuana is legal for purchase, even though cannabis remains classified as an illegal narcotic under U.S. law.

The marijuana market in California alone, which boasts the world’s sixth-largest economy, is valued by most experts at several billion dollars annually and is expected to generate at least a $1 billion a year in tax revenue.

Most California jurisdictions are sitting out the highly anticipated New Year’s Day inauguration of recreational cannabis sales. Manu cities including Los Angeles and San Francisco, will not be ready for days or weeks because of additional red tape required by city and county governments before would-be retailers can obtain their state licenses.

Among the very first will be the Oakland-based Harborside dispensary, which has long ranked as the largest U.S. medical marijuana outlet. It planned to opens its doors at 6 a.m. local time today.

Customers in the recreational sector, which state regulators prefer to call the “adult use” market, are only permitted to buy an ounce (28 grams) of raw cannabis or its equivalent at a time.

Medical patients can buy unlimited quantities, but must present a doctor’s note and have purchased a medical ID card.