Dr. Jeffrey Raber and colleagues at the Werc Shop in Pasadena have published a paper in the journal Natural Products Chemistry and Research showing that the average joint smoker inhales a little more than one third of the cannabinoids present in the plant material prior to burning. The paper is entitled “The conversion and transfer of cannabinoids from cannabis to smoke stream in cigarettes.” 

Raber et al  found no difference in the rate at which THC and CBD are inhaled. They suggest that smoking should not be ruled out as a delivery system for dosed cannabinoid medicine.  Here are their conclusions:

In this study, the transfer of cannabinoids from plant material to smoke stream was investigated. Investigated ranges in the plant material were 4.5-18.6% THCmax and 2.7-7.6% for CBDmax. All cannabis cigarettes contained only cannabis, contrary to some previous studies in which cannabis was blended with tobacco.
Although the plant material used for this experiment contained predominantly cannabinoid acids, mainly neutral cannabinoids were recovered. Less than 0.5% of the originally present THCmax or CBDmax was recovered as carboxylic acids in the mainstream smoke, sidestream smoke and ash. This indicates almost complete decarboxylation of the cannabinoid acids during cannabis smoking.
Recovery of THCmax as THC in the smoke stream varied from 27.5 to 46.3% with an average of 36.9% over all the experiments. The recovery of CBDmax as CBD showed similar results with a range of recovery from 29.9-42.6% with an average of 38.4%. There was no statistically (p=0.05) significant difference between the CBD and THC recoveries. The recoveries in the smoke stream are higher than previously reported levels, which were in the 16-23% range. We expect this is the result of differences in the plant material, smoking protocol and collection methods.
No conversion of CBD to THC was observed in this experiment as indicated by similar recoveries for THCmax in the presence and absence of CBD as well as the consistency of ratios in cigarette versus smoke in different blends. This is in conflict with the results reported by Mikes et al., which suggest THC might form from CBD upon smoking [5]. It could be that this effect only occurs in tobacco-based cigarettes, as tobacco smoke is significantly more acidic than marijuana smoke [11], and acidic conditions are known to convert CBD to THC 
We showed that THC/CBD ratios remained constant upon combustion. This indicates that specific ratios of THC to CBD can be attained by blending a high THC with a high CBD variety. This could be of importance for future studies in which the interaction between THC and CBD is investigated.
On average over all of the experiments, 9.2% of the THCmax could be recovered as THC in the sidestream smoke and 5.6% could be found in the ashes. For CBDmax 11.0% could be recovered in the side stream smoke and 4.1% in the ash. When variations between replicates are considered, there is no significant difference between these values. This indicates a similar behavior for THC and CBD.
On average a little over 50% of the THCmax and CBDmax present in the plant material could be recovered in mainstream smoke, sidestream smoke and ash. Experiments with a third inline trap on either the smoke stream or sidestream side indicated that >90% of the cannabinoids got trapped in the first two gas traps (data not shown) and therefore the missing mass balance cannot be explained by incomplete trapping. A minimal amount of tubing was used and recovery experiments in which we extracted the tubing revealed only trace amounts of cannabinoids. The walls of the enclosure have also been tested, but the cannabinoids present did not influence the mass balance gap significantly. The HPLC chromatograms did not reveal any major unknown peaks that would be able to explain the gap in the mass balance either. This leads us to conclude that the missing cannabinoids have been degraded by combustion to some unknown compound(s).
This data demonstrates that doctors and patients can utilize blended cannabis material to provide different dosing regimens for inhalation that can be tailored to meet each individual patient’s needs even allowing for cannabinoid ratio combinations that may not be present in nature.