Recreational marijuana sales are underway in Colorado, attracting a new type of tourism to the state. Govbeat's Reid Wilson breaks down four things pot tourists should know about the new laws. (Davin Coburn/The Washington Post)

On Wednesday, Colorado became the first state to allow legal sales of marijuana for recreational use. Washington state will do the same this year, and other states might follow suit. Here are answers to some basic questions about marijuana, its effects on the body and the issues raised by legalization:

How does marijuana work?

Marijuana’s main active ingredient is delta-9-tetrahydrocannabinol, or THC, found in the leaves, stems, seeds and especially the flowers of the cannabis plant. It binds to the surface of nerve cells in high-density areas of the brain involved in memory, thinking, concentration, feelings of pleasure, coordination and movement, and sensory and time perception. THC stimulates this communication network, resulting in a marijuana “high.”

What are its medicinal uses?

About 20 states and the District allow the use of marijuana for medical purposes. Most patients seek the drug for controlling pain from medical conditions including cancer, nervous system diseases, glaucoma and migraine. It is also being used to treat nausea and improve appetites of people with HIV or chronic illnesses.

What’s the difference between smoking marijuana and consuming it in food, powders or liquid extracts?

When marijuana is smoked, THC moves almost immediately from the lungs to the bloodstream and to the brain. The effects can last one to three hours. If it’s eaten, say in brownies or cookies, it can take 30 minutes to an hour to have an effect, but the high can last up to four hours.

What are the health effects of marijuana use?

In the short term, it can lead to a rapid heart rate, increased blood pressure, red eyes, dry mouth, increased appetite and slowed reaction time. Long-term use has been linked to impaired thinking, memory problems, panic attacks and other psychological issues. There have also been studies showing a weakened immune system and, for those who smoke the drug, impaired lung function.

How much marijuana is safe to use? Can you overdose?

There is no recorded case of someone dying from an overdose of marijuana, but it has been a factor in accidents or medical issues that can lead to death.

The concentration of THC in seized samples of illegal marijuana has been increasing over the past 30 years, with the average potency more than doubling since 1998, leading to concern about the consequences for young users. But scientists don’t know much about the effects of higher concentrations on the body and brain.

Is it addictive?

It can be. Long-term use, especially by those who start at a young age, has been shown to lead to addiction, with an estimated one out of 11 people who use it becoming dependent on it. Marijuana was responsible for 4.5 million of the 7.1 million Americans dependent on or abusing illicit drugs, according to the 2010 National Survey on Drug Use and Health published by the Department of Health and Human Services. Withdrawal symptoms are similar to those of withdrawal from smoking and include irritability, sleep difficulties and anxiety.

How does marijuana use affect the young?

Last year, Canadian and American researchers reviewed more than 120 studies examining cannabis and its effects on the teenage brain. They found strong indications that early marijuana use can alter development and contribute to mental health problems later in life.

“When you smoke marijuana, you cannot memorize or learn as you should,” Nora D. Volkow, director of the National Institute on Drug Abuse, said recently on NPR’s “The Diane Rehm Show.”

“The other issue is that, the younger you start smoking marijuana, the higher the risk not only to becoming addicted to marijuana, but it also appears to increase the risk for addiction of other drugs in adulthood.”

Volkow added: “What we know about the marijuana, as well as other drugs, is that the effects of drugs in the human brain are not the same when you take them as a child, adolescent or as an adult, and this is because there are significant changes in the brain as we go in the transition from childhood into adulthood.”

How does marijuana use affect driving?

One of the key questions Colorado lawmakers had to wrestle with in setting up a legal marijuana market: When is someone too stoned to drive? The answer isn’t simple.

Prosecutors and some state lawmakers have long sought strict blood-level limits for THC. Many marijuana advocates argue that the drug affects people differently and that setting a hard limit could lead to wrongful DUI convictions. They also argue that, unlike with alcohol, traces of the drug remain in the bloodstream long after an individual has smoked pot. Officials in favor of blood-level limits say tests can pinpoint “active” THC in the bloodstream in the hours immediately after marijuana usage.

Studies have shown that smoking marijuana tends to affect spatial perceptions. Drivers might swerve or follow other cars too closely. They can lose concentration and have slower reaction times. Such findings have led some researchers to conclude that driving while high greatly increases the chances for an accident, and that smoking pot and drinking before driving is a particularly dangerous mix.

Every state forbids driving under the influence. But convictions in drugged-driving cases generally rely on the observations and testimony of police officers rather than blood tests. The White House, in a drug policy paper last year, called on states to adopt blood-limit laws in an effort to reduce drugged driving, but states continue to take different approaches.

Last year, Colorado lawmakers approved a bill that creates a “permissive inference” that someone with a certain level of THC in their blood is impaired. Drivers suspected of driving while high generally would have to consent to have their blood drawn, and they could lose their license if they refuse.

Should you use marijuana if you’re pregnant?

A number of studies have shown that babies born to some women who regularly used marijuana had an increased risk of cognitive and attention deficits, memory and learning problems, low birth weight, pre-term delivery and other issues. But more research is needed to figure out to what extent environmental factors played a role in these studies.

Brady Dennis and Ariana Eunjung Cha