SirWe thank Drs Nordstrom and Hart (2006) for theircomments, because it gives an opportunity to addressan issue of growing importance in experimental cannabisresearch: the comparability of THC dosing across experi-mental performance studies. In general, researchers haveemployed two types of units for reporting the mean amountof THC delivered to subjects when smoking an experimentalcannabis cigarette: (1) %THC contained in a cannabiscigarette and (2) absolute mass of THC (mg). The formerprovides only a relative measure of mass THC that cannotbe used for comparison between studies that usually employdifferent sizes/weights of cannabis cigarettes. The latterprovides an absolute measure of total amount of THCcontained in a cigarette and is much to be preferred overrelative units, because it does allow comparison of THCdose across studies. However, even comparison betweenabsolute units of THC doses can be misleading becauseof variations in smoking procedures that are beingemployed across studies. It has been amply shown thatvariations in puff volume, number of puffs, and breathholdduration produce dose-related changes in plasma levels ofTHC (Azorlosa et al, 1995). Therefore, the best approach forincreasing comparability between studies is to report THCdose as well as plasma THC concentrations. Unfortunately,assessment of plasma THC concentration is not standardpractice in experimental performance research. However, ifit were, it would have helped to evaluate the claims byDrs Nordstrom and Hart that THC doses in the studiesby Hart et al (2001) and Ramaekers et al (2006) werecomparable and that experienced cannabis users aretolerant to the impairing effects of THC on cognition.We will illustrate this point.In their original publication, Hart et al (2001) did notindicate the total amount of THC that was delivered to theirsubjects by smoking. They merely indicated that theirhigh concentration cigarettes contained 3.9% THC of whicheach subject smoked three standardized puffs. Each puffconsisted of 5s of inhalation, 10s of breathhold, and 40s ofexhalation and rest. They failed to report the total massof the cannabis cigarette. Nordstrom and Hart (2006)have now corrected this omission by stating that eachcigarette weighed 1g, on average of which subjects smokedthree-quarters, within a 3min period. This, they argue, isapproximately 30mg of THC which is close to the largestdose (35mg) in the study by Ramaekers et al (2006).It is difficult to see, however, how subjects in the study byHart et al (2001) could have smoked three-quarters of acigarette in only three standardized puffs, when subjectsin the study by Ramaekers et al (2006) needed about 25standardized puffs, that is, 4s of inhalation, 10s ofbreathhold, and 15s of exhalation and rest, to finishsmoking a cannabis cigarette completely. These compara-tive data on THC dosing regimens certainly do not supportthe claim by Drs Nordstrom and Hart that the totalamount of THC delivered to subjects were comparable inboth studies. Of course, the final confirmation of theirclaim can only come from comparative plasma THCdata. These however are not available, as Hart et al (2001)did not assess plasma THC. In addition, their claim is atodds with a previous conclusion by Hart et al (2002)that the behavioral effects of smoked cannabis containing3.1% THC is similar to the effects of 20mg oral THC.Again, no plasma THC data were provided but it is wellestablished that bioavailability of oral THC is only 10% andgenerally three times lower as compared to smoked THC(McGilveray, 2005; Ohlsson et al, 1980). This seems toindicate that THC concentrations during the smokedcondition in this and perhaps other experiments may havebeen very low as well. Of course, this is very speculative butit goes to show how data on plasma THC concentrationswould have come in very handy.