Theoretical limits of the evaluation of drug concentrations in hair due to irregular hair growth.

When examining concentration relationships of doses it must be taken into account that hair growth is irregular. Hair growing from the shaved skin after a single dose of a certain drug cannot possibly contain the same concentration as hair after the same dose that has not been cut over a long period. Concentrations can even change during the hair growth in cases where the hair had been cut a couple of months before the hair sample was taken. The variations in the expected concentrations can exceed 20%. On the other hand, the evaluation of a hair tuft which has grown after the last drug consumption may be important in forensic cases where the hair which has grown earlier is not available. This may lead to misinterpretations at low concentrations. Expected concentrations are calculated assuming a telogen part of 10%.