Home » Module 2: Drug Testing: A Hair-Brained Idea! » Content Background: Binding of drugs to hair

Content Background: Binding of drugs to hair

In order for drugs1 to become incorporated into the hair, they must enter the follicle2. This occurs by two routes. One route of entry is from the capillaries surrounding the follicle. The other route of entry is through the sebum3 secreted by the sebaceous glands connected to the follicle (drugs can enter the follicle from sweat as well). Most drugs will have little trouble leaving the capillaries since there are many pores or fenestrations that allow their passage. Once the drugs get into the follicles and the hair cells, the pH of the hair cell environment will determine if the drugs can get out. For example, when drugs that are weak bases4 are in an acidic environment, they tend to exist in a predominantly charged form (they accept a H+ from the donor) (see Module 1). This is the case for drugs such as nicotine, morphine, cocaine and amphetamine. These drugs are weak bases and they bind to the melanin5 because it is acidic. The nature of the bond between oppositely charged atoms is electrostatic. Electrostatic bonding is a very common form of bonding between drugs and proteins (either enzymes6 or receptors7). When drugs bind to receptors on neurons for example, they change the electrical activity of those cells. However, in hair, the binding of drugs to melanin does not affect the cell’s function. Instead, the weak bases become trapped inside the hair cells because they exist mainly in their charged form. As charged molecules, they have difficulty diffusing through the lipid membranes of the hair cells to reach the capillaries. The more melanin present in the hair, the more binding of weak base drugs. So, for the same dose of drugs such as cocaine and morphine, higher levels of these drugs are present in black and brown hair compared to blond hair. Non-Caucasian races with black hair have higher concentrations of these drugs in their hair compared to Caucasians. After a single dose, the drugs remain in the hair from 2-8 months (see Figure 7), depending on the individual. Drugs such as cocaine can even be detected in the hair of newborn babies after use of cocaine by the mothers during the 2nd and 3rd trimesters of pregnancy. Washing of the hair does not eliminate the drug, although anything that destroys the pigment will decrease the binding of the drug.

1 a substance that affects the structure or function of a cell or organism.
2 a small sac; hair follicles are internalized structures of epithelial cells in which the hair is synthesized and grows
3 a fatty substance made in and secreted by sebaceous glands attached to hair follicles
4 a compound that tends to accept a H+ when placed in an acidic solution
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 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.
7 a protein to which hormones, neurotransmitters and drugs bind. They are usually located on cell membranes and elicit a function once bound.


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

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