Deciding between CBD or THC for pain is not an easy process. This is, first of all, because pain is a complex symptom with a myriad of possible causes—no one treatment works for every kind of pain. Also, so far, investigations have focused mostly on either a combination of tetrahydrocannabinol (THC) and cannabidiol (CBD) or only THC. So the evidence is limited. To complicate matters even more, it appears that different cannabinoids could be helping for different kinds of pain.  
In the meantime, people continue to use medical marijuana and CBD-rich hemp products to alleviate physical pain, as they have done for centuries. Recent statistical data are showing that approximately 95 percent of medical cannabis patients in states like Oregon and Colorado are using it to treat severe pain. So, as legislation and attitudes change—and as the need for an alternative to opioids and anti-inflammatories increases—this number is growing ever faster.  
Let's unpack the available evidence to see whether the need to choose either CBD or THC for pain is valid.
As mentioned, research on the use of either CBD or THC for pain modulation is either limited or inconclusive, partially due to the difficulties in getting federal approval to research cannabis. The design was also problematic in some studies. It's clear, though, that more robust, well-designed clinical trials and studies are needed before guidelines can be standardized regarding (for instance) indication, product dosage, potency, and formulation.
In an extensive but slightly dated literature review (2006) on the subject of cannabinoids and pain regulation, the authors concluded that phytocannabinoids and their synthetic analogs could be highly efficacious for treating various pain conditions. Yet the effects of natural cannabinoid extracts seem to be better than those of synthetic cannabinoids, according to the authors.
For this reason, it would be a good idea to experiment with full-spectrum CBD oil or medical cannabis to address especially serious pain conditions. THC is mostly extracted from marijuana, while hemp is commonly richer in CBD.
As noted, research furthermore suggests that one cannabinoid may be more helpful for certain types of pain than the other. For instance, CBD, a CB1 and CB2 receptor agonist (meaning it doesn't bind directly to cannabinoid receptors in the body) seems to be more effective to ameliorate neuropathic pain. This is the type of chronic pain associated with conditions such as multiple sclerosis and fibromyalgia. 
That said, for most pain conditions, it appears that THC and CBD together are a formidable team.
Any dose of CBD and low-to-medium dose THC is considered safe, and users can experiment on their own.
Marijuana and THC use should be avoided completely in the case of schizophrenia or similar delusional disorders, but taking CBD would be safe. Consuming very high doses of both cannabinoids is associated with some side effects, but these normally self-correct. High-dose CBD can cause unsafe drug-drug interactions, so it would be wiser to consult a doctor before use if you are taking prescribed medication.
For many people, the term “THC” is interchangeable with the word “cannabis.” Cannabis, however, encompasses a variety of strains, and each has different effects. Marijuana and hemp (two cannabis varieties) each contain over 100 cannabinoids that work with the human body’s endocannabinoid system.
Historically, THC is better known because of its psychotropic effects. Commonly reported symptoms of use include euphoria, wellbeing, and relaxation, but also paranoia, anxiety, and fear in some individuals. 
CBD triggers a different sensation. In fact, it causes no noticeable shift in perception or mood—no fear, no anxiety, no euphoria. Its effect is most commonly described as a gentle sense of relaxation, well-being, and calmness.
THC and CBD interact with the endocannabinoid system in very different ways. THC directly activates the body's cannabinoid receptors, particularly the CB1 receptors concentrated in the brain and throughout the central nervous system.
The THC molecule locks into the CB1 receptor like a key, allowing it to affect—among other functions—memory, mood, and nociception.
CBD's action at receptor sites is quite different. Instead of interacting directly with either the CB1 or CB2 receptors, it seems to have an indirect effect. The majority of CB2 receptors are found on white blood cells in the spleen and tonsils. This effect inhibits the action of some cannabinoids (such as THC), as well as inhibiting the reuptake of others. It also interrupts receptor activity, such as delaying anandamide degeneration. Anandamide is an endocannabinoid, meaning it is produced by the body itself. 
Other cannabinoids, called terpenes, also seem to play a role in pain management. While the interactions among terpenes, THC, and CBD are only partially understood, their ability to target pain when taken as whole-plant medicine is very promising.
Don’t forget that there are many different ways to ingest CBD or cannabis, far beyond smoking the straight flower. These days, cannabis and CBD products include:
For chronic, severe pain associated with terminal diseases, epidurals, rectal suppositories, skin patches, and intravenous drips will probably be good options. An epidural is favorable because only tiny amounts of cannabinoid extract are necessary for excellent effects. 
There are a few things to keep in mind when picking a cannabinoid for your specific needs:
A recent report published in the Journal of the American Medical Association presents evidence suggesting that opioids are no more effective for the treatment of chronic pain than, for instance, ibuprofen. 
Yet the biggest problem with CBD or THC for pain? This is probably the industry itself, which is largely unregulated. Also, the plant continues to be demonized. This is a great pity, if not a crime of sorts. As noted by Sanjay Gupta, M.D., who wrote in a very frank article: “[Marijuana] doesn’t have a high potential for abuse, and there are very legitimate medical applications. In fact, sometimes marijuana is the only thing that works. … [I]t is irresponsible not to provide the best care we can as a medical community, care that could involve marijuana. We have been terribly and systematically misled for nearly 70 years in the United States, and I apologize for my own role in that.” Never exchange your current pain or other medicine for marijuana or CBD without your doctor's approval. 
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