The stability of drugs within hair depends on how the hair structure interacts with the drug substance. Key factors determining this interaction are the physical structure of the hair and its chemical nature.

Hair Drug Stability Factors

The stability and retention of drugs in hair are influenced by several properties:

• Morphology of hair (D): The physical structure of the hair shaft, including the cuticle layer, cortex, and overall surface texture, affects how a drug can penetrate, bind to, or be trapped within the hair. Variations in hair morphology create different environments for drug interaction.
• Physicochemical properties of hair (E): These include aspects like hair porosity, surface charge, diameter, and the chemical composition of the keratin and cuticle. These properties dictate the drug's solubility, binding affinity, and potential for degradation or leaching within the hair matrix.

Factors Not Directly Related to Hair Stability

Other listed factors are less directly related to the inherent stability of a drug within the hair structure itself:

• Eversion Factor (A) typically relates to drug delivery device performance.
• HRP-TMB Complex level (B) relates to specific biochemical assays.
• Oxyphermelanin concentration (C) relates to melanin type, which is a component of hair but less general than overall morphology and physicochemical properties.

Therefore, the primary factors governing drug stability in hair are its morphology and physicochemical characteristics.