Home » Module 2: Drug Testing: A Hair-Brained Idea! » Content Background: How do drugs get into hair from the environment?

Content Background: How do drugs get into hair from the environment?

When drugs1 are smoked or inhaled as a vapor, a portion of the smoke or vapor escapes into the air surrounding the user or others nearby. To be dissolved in the smoke or vapor the drug must be heated to a high temperature in its uncharged (lipophilic2 or lipid-loving) form. This is especially the case for weak bases3. For example, nicotine and cocaine are smoked in their uncharged or free base4 form (see Module 1). The same is true for heroin or opium smokers. Because the uncharged forms of these drugs are lipophilic, they can easily penetrate the hair shaft (that part of the hair above the skin). Once in the hair shaft, the drugs can bind to melanin5 as described in the previous section. One way to distinguish between drugs entering the hair from the body vs the environment is to look at the pattern of drug binding along the hair itself. With very recent drug use, the drug will be present in the hair root (the drug can be found in the hair root as early as 8 hours after drug use), but not in the hair shaft above the skin. With past drug use, the drug will be located in a small section of the hair at a distance from the root proportional to the number of months after the drug use (hair grows at 0.35 mm/day or about 1 cm/month). Experimental subjects given low doses of cocaine show this pattern of cocaine binding to hair (Figure 7). In contrast, environmental exposure will produce no real pattern. The length of hair that is exposed to the air during the time of exposure will uniformly bind the drug.

Figure 7 Pattern of cocaine binding to hair from a subject who snorted 1 low dose of cocaine. From: Henderson et al. (1996). J. Analytical Toxicology, 20: 1-12.


Another way to distinguish the route of entry of a drug into the hair is by analyzing the hair for the drug metabolites6. All drugs must go through the liver as they circulate through the bloodstream. In the liver, most drugs are metabolized by enzymes7 to inactive products. Their structure is changed to make them a little more polar8 (charged) so that it is easier to eliminate them in the urine (charged compounds get trapped in the urine and can’t get back into the bloodstream). As the metabolites circulate throughout the bloodstream some of them stay in the kidney to be excreted, and some of them reach other parts of the body. If the metabolites are not too polar, they can enter hair follicles9, just as the parent drug. So cocaine and some of its metabolites can be found in hair and produce the same pattern of accumulation. If the cocaine were to enter the hair through a vapor in the surrounding environment (i.e., second hand smoke), there would be no metabolites present in the hair.

1 a substance that affects the structure or function of a cell or organism.
2 high lipid solubility. Lipophilic compounds dissolve readily in oil or organic solvent. They exist in an uncharged or non-polar form and cross biological membranes very easily.
3 a compound that tends to accept a H+ when placed in an acidic solution
4 the unionized form of a weak base. With reference to cocaine, it is the smokable form.
5 the pigment found in hair, skin, feathers, etc.. It is a biopolymer found in granules within melanocytes and transferred to keratinocytes to provide hair color.
6 usually an inactive form of a drug or other substance that is more polar (charged) than the parent compound. Drugs are metabolized by enzymes primarily in the liver.
7 a protein that catalyzes the rate at which a reaction occurs. It binds to one of the reactants (a substrate) to cause a change in the reactant’s structure, facilitating the reaction.
8 a chemical property of a substance that indicates an uneven distribution of charge within the molecule. A polar substance or drug mixes well with water but not with organic solvents and lipids. Polar or charged compounds do not cross cell membranes (lipid) very easily.
9 a small sac; hair follicles are internalized structures of epithelial cells in which the hair is synthesized and grows