This shampoo is designed for scalps that become oily, flaky and irritated, so frequency should depend on how quickly flakes and oil build up for you. A sensible starting point is two to three times a week, replacing one of your regular shampoos with this one. If your scalp remains comfortable and fresh between washes, there is no need to use it more often. If oil returns quickly, you may wash more frequently, but avoid going from a gentle routine straight to daily use; skin that is already reactive may need time to adjust.
According to the product description, this shampoo can also be used as a targeted wash from time to time rather than as an everyday hair product. That means you do not have to make it your only shampoo. You can alternate it with a mild everyday shampoo, especially if you have dry ends or use styling products.
For the first few uses, pay attention to how your scalp feels after rinsing and the next morning. A slight freshness is normal. If you notice tightness, redness or a strong stinging sensation, stop using it and let your scalp settle before trying again. If your skin is very sensitive, patch test on a small area behind the ear before using it over the whole scalp.
Wet hair thoroughly, massage a moderate amount into the scalp rather than only the lengths, and leave it on for one to three minutes before rinsing. This gives the zinc sulfate and birch-derived oil time to work on the scalp surface. Rinse completely and avoid applying near the eyes.
If scalp symptoms persist, are severe or worsen, the most reliable step is to ask a pharmacist or doctor rather than continuing to adjust the shampoo on your own.