Many of us have heard - or know from experience - that cannabis can make the symptoms of anxiety and fibromyalgia more bearable, even assuage the agonies of chemotherapy and epilepsy. But what's the science behind that relief?

Civilized asked Dr. Jahan Marcu - the Chief Scientist at Americans for Safe Access, and a court-qualified expert on cannabis and contributor to The Journal Of Cannabis In Clinical Practice, Project CBD, the Philadelphia Examiner - to shed a little light on exactly how cannabis can treat pain and other symptoms connected to serious health conditions.

How long have we known cannabis can be used to treat pain?

We owe a great deal of our insight into the treatment of pain to the plant world. Take willow bark, which led to the development of Aspirin, or the opium poppy, the prototype narcotic for pain relief. Cannabis, too, has helped fight pain for thousands of years - probably since before written history. But we have Egyptian papyrus that mentions medical uses for cannabis, tons of ancient artifacts and jars containing cannabis, even records of kings treating their children for a condition we now believe to be epilepsy with cannabis. There is a ton of historical evidence - and recall, it has only not been deployed as a medicine in the U.S. since the 1930s.

How do THC and CBD interact with pain?

With the isolation of THC, scientists discovered the endocannabinoid system, which modulates both pain, and the receptors that sense pain. Take the so-called "runner's high": that's what happens when the endocannabinoid system is stimulated, leading to the release of endorphins.

In terms of scientific research, there have been studies involving over 1,000 participants looking at smoked, vaped, whole-plant cannabis, and synthetic THC. They've looked at HIV patients, neuralgia, post-operative pain, spasticity, fibromyalgia, chronic intractable pain, arthritis, cancer - and, overwhelmingly, the research shows cannabis preparations can be useful. In controlled trials, smoked cannabis has been shown to be very effective for HIV neuropathy and pain. Pure THC and its derivatives - including synthetic versions - have been used for postoperative pain and fibromyalgia, spinal cord injuries and MS. Chemotherapy makes people very sensitive to pain, and cannabis helps with that. Chronic pain states are postulated to arise, at least in part, from a dysregulation of the cannabinoid system.

When you administer the appropriate combination of THC or CBD, you regain the control of pain. The bottom line: it works for a variety of conditions.

How does cannabis stack up against traditional pain relievers?

Cannabinoids have been demonstrated to work well with opioids: one human study demonstrated that if you give someone one-quarter of the dose of opiates in combination with cannabis, you get the same effect as with a larger dose of opiates with no cannabis.

One thing about cannabis, however, is that people can test positive for it a month or more after their last use. That means you could still get popped by a zero-tolerance drug test weeks after your last use. When you take cannabis, which has lipid-like qualities, your body takes it into your fat, and it's time-released - which is also why researchers have a hard time demonstrating cannabis withdrawal. You get a longer, lower effect.

What about addiction?

Definitely compared to nicotine, cocaine, and opiates, people who use cannabis have a very, very low chance of developing dependence. People talk about "cannabis use disorder", not "addiction" because dependence is so hard to observe and takes so long to manifest. Some doubt whether it even exists. In lab trials, they couldn't get animals to self-administer THC.

Real addiction - in which people are living just to get the drug, rock-bottom addiction - basically never happens with just cannabis. Sure, you can develop a habitual use disorder - but it'll take a lot of cannabis, and a long time.

Any tips for those who haven't done this before?

Inhalation is not only the most common method to administer medicine, but it's also the most effective if people are self-administering. If you have no experience with cannabis at all or are nervous about side effects, start with a 1-to-1 ratio of THC to CBD: CBD will block the intoxicating effect of THC. The two preferred methods of ingestion in research studies are inhaling vapour, and under-the-tongue, rapid-absorption spray.

If you can't stop vomiting from chemo you can get a pill to help you with that, but it doesn't help if you can't keep it in your stomach, you know? With inhalation, you can control the dose and you don't get side effects. The effects of cannabis consumed in pill form can come on suddenly to inexperienced users, and take anywhere from a half-hour to hours to kick in. In state-sanctioned programs, you can get dosage info with your product: otherwise, it's a trial and error thing.

Follow Dr Jahan Marcu on Twitter.