Curtis Swift, Ph D., Mesa Lavender Farms
The inverted U-shaped dose-response curve helps explain why a dose of 15 mg of CBD (cannabidiol) can be effective while doubling the dose or taking a lesser dose of CBD is less efficient. There is a sweet spot based on body weight and milligrams (mg) of the CBD dose.
CBD conforms to typical bell-shaped dose-response curves also known as a reverse U-shaped dose response. This appears to be true no matter which animal is tested. Researchers tested this hypothesis with men and women in public speaking tests and found the bell-curve effect was accurate. 
In these two studies, CDB at 150, 300 and 600 mg produced an inverted U-shaped dose-response curve. In other words, the lowest or highest doses of CBD had little or no effect on anxiety and a significant response was only achieved with an intermediate dose, the sweet spot. This dose-response pattern is consistent with evidence from CBD animal studies within a limited range of doses. Both studies used CBD in an oral gel caps which were swallowed.
When considering the amount of CBD used in these two studies keep in mind the bioavailability (i.e. effectiveness) of oral CBD in humans is around 6% , and when taking CBD under the tongue the bioavailability is much greater.  Sublingual (under the tongue) delivery produces superior bioavailability compared to oral dosage forms of CBD.  Higher oral doses are required to produce the same results when doses are taken under the tongue.
Here is a good comparison: If you take 15 mg of Mesa Lavender Farms CBD oil under your tongue, you will need to take 4 to 6 times that amount of CBD in a capsule or drink of the same concentration.
CBD abides by the law of diminishing returns with the inverted U-curve: More is not necessarily better. At a low dose, you don’t see the intended effect. At a moderate dose, you do see the intended effect. And at high doses, you go back to seeing no effect.
Mesa Lavender Farms CBD oils are formulated so one drop contains one milligram of CBD. We recommend 1 milligram under the tongue for 30 to 90 seconds per 10 pounds of body weight per day. Thus, a person of 150 pounds would take 15 drops of Mesa Lavender Farms CBD oil sublingual, while a 200-pound person would take 20 drops of Mesa Lavender Farms CBD oil to reach the sweet spot. Since everyone’s metabolism differs, slight adjustments may be needed in increment of 2 to 5 drops until the Sweet Spot is reached. As your metabolism changes as you age, the dosage will also need to change.
CBD dose-response curve references: