SLS is the original surfactant that made shampoo foam the way most people expect. It’s also why a lot of people have reacted badly to shampoo without knowing why — SLS is in the top ten most common contact allergens tracked by dermatological patch testing.
On the hair, SLS is indiscriminate: it strips dirt, product, pollution, and the protective lipid layer that keeps your cuticle closed and your color locked in. That’s the trade for the lather. If your scalp flakes, itches, or burns after washing, there’s a reasonable chance SLS is the cause.
SLS is still common in drugstore shampoos and cheaper body washes. Most premium brands have moved to SLES — milder but still a sulfate — or to sulfate-free surfactants entirely.