Pain: chronic/ acute pain

One of the major symptoms that cannabis (smoking or the oral) is used worldwide in treating "chronic pain". Cannabinoids (via ECS and other pathways) modulate pain-perception and heal tissue injury. There are 3 types of pain: Nociceptive pain, neuropathic pain and central pain.

Nociceptive pain usually originates in the peripheral tissues due to tissue damage. Chronic inflammation leads to tissue damage (Joint destruction in Rheumatoid Arthritis etc.). These signals are carried via ner fibres to the brain center where pain is perceived and processed. Following this, the brain sends action signals to the affected part (like tightening of muscles etc.)

Pain-killer drugs affect the pain pathways in different ways. Some cause local numbing so the pain felt is reduced but the inflammation is still there. Some class of drugs (COX inhibitors or prostaglandin inhibitors) reduce inflammation but these drugs usually come with a heavy side-effect profile which makes long-term use very difficult.

For rheumatoid arthritis, there are disease modifying drugs available which are immunomodulators but again their side-effect profile causes many people to opt out of the treatment.
While nociceptive pain has survival value (due to its warning and defensive properties), both Neuropathic and central pain are due to dysfunctional pain signals where the disease involves the pain interpreting system.

Neuropathic pain is commonly seen in long-standing diabetes mellitus, parkinson’s disease, Alzheimer’s, chronic alcoholism, herpes (shingles) etc. For example in Diabetes there is not tissue inflammation but the nerves carrying pain signals are affected.

In central pain, the pain signals are amplified due to persistent central nervous system dysfunction. A classic example is Fibromyalgia.

Cannabinoids work in various ways to mitigate pain. CB2 receptors present at the tissue levels reduce inflammation thereby reducing pain. THC (tera hydro-cannabinol) has been shown to produce analgesic and anti-hyperalgesic (reducing pain perception) effects in animal models. A meta-analysis study of 28 clinical covering 2454 patients showed that there was a significant reduction in pain among those taking cannabinoids versus those taking a placebo.

In a study researching on central pain, they concluded that vaporized cannabis flower reduced neuropathic pain scale ratings (without being dose-dependent).

There is definitely scope for more research and studies in relation to use of medical cannabis for chronic pain. However, in most conditions like fibromyalgia, MS, RA, Neuropathic pain etc. medical cannabis is better tolerated and shows a lower adverse effect profile compared to conventional treatment

Reference:

Whiting PF, Wolff RF, Deshpande S, et al. . Cannabinoids for medical use: a systematic review and meta-analysis. JAMA. 2015;313:2456–2473.

Wilsey BL, Deutsch R, Samara E, et al. . A preliminary evaluation of the relationship of cannabinoid blood concentrations with the analgesic response to vaporized cannabis. J Pain Res. 2016;9:587–598.
Cannabis and Pain: A Clinical Review. Kevin P et al. Cannabis Cannabinoid Res. 2017; 2(1): 96–104. Published online 2017 May 1. doi: 10.1089/can.2017.0017