Ketoconazole
In plain terms
An anti-fungal used in dandruff and anti-flaking shampoos, and in creams for fungal skin infections. It slows the scalp yeast behind dandruff and seborrheic dermatitis. The same drug also exists as a pill, which is a different story.
What the evidence shows
Ketoconazole is an effective anti-fungal, and the serious warnings written about it describe the swallowed pill rather than the shampoo or cream. Taken by mouth, ketoconazole can injure the liver, lower testosterone, and suppress the adrenal glands, and in 2013 the FDA and European regulators sharply restricted the oral drug for those reasons. On skin and scalp it works differently, because it is barely absorbed: measured after months of shampoo use two to three times a week, it left no detectable trace in the blood, and the liver, hormone, and adrenal effects seen with the pill have not shown up with topical use. So the reason this lands at depends is not the oral drug's problems, which the rinse-off does not deliver; it is that ketoconazole is a recognized contact allergen. Some people become sensitized to it and react with an itchy rash where it is applied, a reaction documented well enough to separate this from a no-concern ingredient. For most people the shampoo and cream are well tolerated and effective when used as directed; for the minority who react to it, it is one to recognize and avoid. The serious warnings attached to the name belong to a medicine you take, not one you rinse out of your hair.
On the label
Listed as ketoconazole among the active ingredients, in a Drug Facts panel since it is a drug. The liver and hormone warnings you may read about apply to ketoconazole tablets; the barely-absorbed shampoo and cream do not carry them, and the real topical caution is that some people are allergic to it.