wait a minute, there are three active ingredients used here:

>zinc pyrithione
used in standard formulation head and shoulders
anti-fuingal agent

"Head & Shoulders active ingredients do this by affecting a particular microbe, a yeast-like fungus called malassezia, which has been identified as the root cause of dandruff."

>selenium sulphide
Used in clinical grade formulation of head and shoulders

>Ketoconazole
used in Nizoral. they claim it's 10x more effective at fighting dandruff than head and shoulders. it's not clear if this is targeting the fungal variants of dandruff or dry scalp.