Pure CBD oil contains only an isolate of cannabidiol (CBD). This means the oil is free from other cannabis compounds or additives.
So far, the health benefits of specific CBD isolates have not been investigated much. Based on the current evidence, one can only venture to say that pure CBD oil should offer many of the same benefits as so-called full-spectrum CBD oil. Yet there is one caveat—it will need to be taken in exactly the right dose.
This has to do with what is called the bell-shaped dose-response, a phenomenon relating to the efficacy of medicines. Read on for more on this, and also for pointers on buying a quality CBD oil.
NB: Never attempt to replace prescription medication with CBD oil without a doctor's consent. Especially for intractable epilepsy, a serious condition, pure CBD oil seems to work most effectively together with other anti-epileptic drugs (AEDs).
CBD's addition to any existing drug regimen needs to be done under medical supervision, especially for serious health conditions.
Using it as a general supplement or to remedy insomnia, anxiety, inflammation, etc., however, should be safe—even recommended. Cannabidiol has not yet been associated with any adverse events, and it has many proven health benefits.
Pure CBD oil's ability to help reduce seizures in very serious epileptic conditions appears to be its only scientifically confirmed and researched-backed benefit to date. Last year saw a first in the U.S. when a natural, CBD-only drug was approved by the FDA. It is prescribed together with other AEDs and is, unfortunately, prohibitively expensive. Its efficacy also depends on the dose, which is mostly patient-specific.
Pure CBD oil's ability to help when no other therapeutic intervention seems to is significant. One case study demonstrated its possible excellent utility in long-standing, super-refractory status epilepticus (SRSE).
SRSE is a rare but very harsh epileptic condition, often untreatable and deadly. In this case, a 12-year-old girl's epileptic seizures were effectively managed with CBD adjunct treatment. The researchers concluded with a recommendation:
"Given the paucity of evidence-based therapies for SRSE as well as the prompt and enduring response that accompanied the adjunctive administration of CBD in this patient, CBD should be a consideration in the treatment of SRSE." 
This is a definite benefit, if drug tests are a concern. Pure CBD oil contains no tetrahydrocannabinol (THC), which is the target chemical of these tests.
Using a CBD isolate, though, many people find that they're underwhelmed by the results. There could be a number of reasons for this.
One of the possible culprits could very well be the dose, and that too little or too much gets ingested.
This is due to a phenomenon called, as mentioned, the bell-shaped dose-response curve of medicine. The "bell-shaped" refers here to the line on an X-Y graph, as it relates to the effect of increased drug doses in an organism or study subject.
In pharmacology, this type of curve shows that the efficacy of a medication increases as the dose increases. Then, however, it reaches a peak (the top of the "bell"), from where efficacy declines even as the dose increases.
This effect is not seen with full-spectrum CBD, as was demonstrated in one oft-quoted animal study.
Conducted at the Lautenberg Center for General and Tumor Immunology in Jerusalem, Israeli researchers looked at the difference between pure CBD extract and highly enriched CBD (named "clone 202" in this study).
The treatment aim was for pain and inflammation in mice.
The CBD-rich extract contained other cannabis compounds, but was nevertheless low in THC.
The researchers observed this:
"In stark contrast to purified CBD, the clone 202 extract, when given either intraperitoneally or orally, provided a clear correlation between the anti‐inflammatory and anti‐nociceptive responses and the dose, with increasing responses upon increasing doses, which makes this plant medicine ideal for clinical uses.
The clone 202 extract reduced zymosan‐induced paw swelling and pain in mice, and prevented TNFα production in vivo. It is likely that other components in the extract synergize with CBD to achieve the desired anti‐inflammatory action that may contribute to overcoming the bell‐shaped dose‐response of purified CBD. We therefore propose that Cannabis clone 202 (Avidekel) extract is superior over CBD for the treatment of inflammatory conditions." 
So, to translate to laymen’s English—the researchers could clearly determine the dose and effect of full-spectrum CBD, which was effective for both pain and inflammation in mice. This effect increased as the dose increased.
In contrast, pure CBD extract demonstrated a bell-shaped dose-response and overall lower efficacy in inflammation.
Irrespective of which CBD oil is preferred, good quality will always trump less expensive but lower-quality oil.
Also, the CBD market is still largely unregulated, so an informed choice will be better to avoid getting scammed into buying a useless product.
As was stressed in one study that compared the content of the oil to what it states on the label:
"Of tested products, 26% contained less CBD than labeled, which could negate any potential clinical response." 
So, simply trusting a label could result in poor outcomes. Fortunately, there are a few things that usually give away the quality of a CBD oil. The buyer can ascertain these for themselves before any money is exchanged.
This information should be available to the public on all the manufacturer and distributor marketing material. (This should be the first clue. If the following info is not readily available—either upon request, on the marketing material, or on the label—it is more than likely not a very good oil.)
RELATED: Are You Using Organic CBD Oil?
Pure CBD oil is not without merit, and it can be used with safety. Some users may experience harmless side effects when taking high CBD doses, but these usually self-correct after a while. The better choice would probably be a full-spectrum CBD oil, though.
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