By accident trying to rid dd of nits, and having run out of special shampoo, we used head and shoulders.
Brilliant, there seems to be something in it that nukes them completely.
I got another dose, bitten even on my neck and shoulders, and the first time I used it I only combed out 4 with the comb and the second time 3. Next they were all gone.
Can only assume that it leaves a residue on the scalp that they don't like.