Does Marijuana Cause lung Cancer?

Marijuana, both recreational and medical, is becoming legal in more states. Even as more people use it, health experts aren't sure whether smoking weed increases the risk of developing lung cancer. Here's what researchers know - and don't know - about the connection.

Why can it be harmful?
The link between tobacco smoking and lung cancer is well known. Studies show that marijuana smoke contains many of the same harmful substances as tobacco and often more. Hazards include:

benzo(a)pyrene
Benz(a)anthracene
phenols
vinyl chlorides
nitrosamines
Reactive Oxygen Species
 

People also smoke marijuana differently than tobacco, which can pose a greater risk to the lungs:

Typically, you inhale marijuana smoke deeply and hold it in, giving the toxins more contact with your lung tissue and a better chance of sticking there.
Normally, you smoke a joint all the way. Tar, the sticky substance that remains after combustion, is rich in pollutants and concentrated at the end of a joint.

When scientists examined the lung tissue of some people who regularly smoked marijuana, they found changes known to signal future cancer growth.

Questions remain
 

Given what scientists already know, why is it so hard to tell how smoking marijuana affects your chances of developing lung cancer? Studies looking for a direct link between the two have had conflicting results – some have found evidence linking marijuana to lung cancer, while other data show little or no link.

The subject is also difficult to study. Scientists say that certain factors limit the reliability of research.

Most research on marijuana dates back to a time when it was still largely illegal. It is difficult to collect information on illegal behavior. Most studies have asked people to report how often they smoked marijuana, and researchers know that these types of surveys, called 'self-reported,' aren't as reliable as when they collect data in other ways. This is because people don't remember their behavior perfectly or may underestimate or hide how often they do something that others think is wrong.

Illegal marijuana, unlike tobacco, has no control over its strength or quality. People don't use the same amount in one 'dose'. This makes it difficult for researchers to set standards for measuring their effects.

Another problem is that many people who smoke marijuana also smoke tobacco, sometimes mixed in the same cigarette. So if they have lung cancer, it's impossible to know what substance caused it.

Some marijuana smokers in the studies were quite young, which could skew the results. Cancer can take time to develop.

On the other hand, most people who use marijuana do not smoke as much as tobacco users, which could reduce their likelihood of having a problem.

Animal studies suggest that certain chemicals in marijuana work against tumor growth, which may explain why lung cancer isn't as common among people who smoke it as scientists think. Studies on this are still in their infancy and researchers need to explore this theory further.

The future
 

Now that marijuana is legal in more places, breeders are making the product more standardized and potent. More and more people smoke.

A link between smoking marijuana and lung cancer is unclear at this time, but researchers have an opportunity to overcome some of the issues that previous studies have obscured.