The entourage effect is, essentially, the idea that cannabis compounds function more efficiently and wholly when combined, in a kind of synergy that the endocannabinoid system understands very well.
Rather than a “one plus one equals two” scenario, users of CBD with THC should see a “one plus one equals four” type of effect–where the benefits of CBD are enhanced exponentially by the addition of trace THC, since it will help the endocannabinoid system utilize the CBD better.
Think of it like migraine medicine: the painkiller is what will ultimately fix your headache, but the caffeine in those pills will help it work better and faster.
Does this mean CBD without THC is useless? Far from it! Growing research on the effects of CBD alone–known as CBD isolate–suggests that you can still see benefit, particularly in insomnia and inflammation.
There are also broad-spectrum CBD products, which contain CBD and other compounds from cannabis, but no THC. These should, in theory, produce a greater effect than the CBD isolate, but not as significant as full-spectrum.
Some users report needing a greater amount of CBD isolate or broad spectrum than full-spectrum products to see results, so keep that in mind when deciding on your dosage and evaluating a product’s efficacy: five drops of isolate might do nothing for you, while five drops of comparable full-spectrum CBD could have profound effects.
Overall, both scientific and anecdotal research suggests full-spectrum, THC and all, is best at achieving noticeable results–but if you can’t or don’t want to ingest THC in any amount, it’s still worth trying CBD isolate or broad-spectrum.