Can I use this shampoo if I am already using another scalp treatment, like a serum or tonic?

You can use it, but careful introduction is important. This shampoo contains active ingredients (selenium sulfide, salicylic acid) that alter the scalp's environment. Adding another leave-on treatment (like serums with acids, retinoids, or stimulating tonics) increases the overall intensity and potential for irritation.

The safest approach is to not start both products at the same time. First, establish a routine with the shampoo alone for 2-3 weeks to see how your scalp responds. If it is tolerating the shampoo well, you can then cautiously introduce the other treatment. Use them on separate days initially—for example, the shampoo one day and the serum the next. Closely monitor for signs of over-drying, stinging, redness, or increased sensitivity. If any discomfort occurs, discontinue the leave-on product and revert to using just the shampoo. The product notes specifically advise introducing other intensive scalp products carefully alongside it.