The model generally proposed to explain the incorporation of drugs into hair is one in which drugs enter hair only by passive diffusion from the blood stream into the growing cells at the base of the hair follicle. However, this model may be over-simplified. More recent experimental findings suggest that drugs may enter hair from multiple sites, via multiple mechanisms, and at various times during the hair growth cycle. A more complex model is proposed in which drugs and metabolites are incorporated into hair during formation of the hair shaft (via diffusion from blood to the actively growing follicle), after formation (via secretions of the apocrine and sebaceous glands), and after hair has emerged from the skin (from the external environment). Further, drugs can be transferred to hair from multiple body compartments or pools located in tissues surrounding the hair follicle. These mechanisms could also be drug-specific. A more precise understanding of the mechanisms involved in the incorporation of drugs into hair is critical for forensic scientists in order to interpret the results of hair analysis properly.