Chapter 12 405 basis for acceptance by the US Food and Drug Administration since then, ZPT shampoo and conditioning rinse-off products have been widely marketed. This category of antidandruff products has been approved for over-the-counter use in the United States for dandruff treatment at 0.3‑2% in shampoo and rinse-off products,60,64,77-78 and 0.1‑0.25% in leave-on products.77-78 The efficacy of these products has been demonstrated in many clinical trials.8,35,60,64,66-67,77-79 While ZPT possesses high intrinsic antifungal activity against Malassezia,35,60,64,66-67,77-78 its practical efficacy is dependent on multiple vectors, including but not limited to particle size, particle shape, deposition amount, coverage, and availability of the deposited material. These parameters can all be varied to deliver optimal efficacy. For example, platelet ZPT at a particle size of 2.5 microns is optimal for deposition on the scalp through shampooing and for providing scalp surface coverage (Table 2).56 Table 2. Optimization of active particle size increases dandruff efficacy of marketed 1% pyrithione zinc shampoos. 1% pyrithione zinc shampoo Avg. particle size (mm) Median Effect Size Range of Effect Sizes Shampoo A 13 0.85 1 study only Shampoo B 5 1.41 [1.36, 2.93] Shampoo C 2.5 2.08 [0.55, 2.14] Shampoo D 0.5 1.74 1 study only Individual effect sizes are standardized mean differences between active and placebo shampoos of the reduction in scalp flaking after six weeks of use. The results were taken from 14 separate studies. Since some pyrithione zinc shampoos appeared together in the same study, the effect sizes were computed accounting for the correlation and for unequal variances. In products containing particulate actives like ZPT, the efficacy is effected by the size and shape of those particles, as these factors affect the amount deposited, the persistence of the deposit during rinsing, and the degree of scalp surface coverage. Clinical efficacy testing has demonstrated that particle size is a significant variable in ZPT- based product efficacy and that not all ZPT-based shampoos can be