Answered by Shaykh Amjad Rasheed
Is it permissible to wash ones hair with shampoo during ones purificatory bath (ghusl)? Or does one have to do it after ones ghusl?
It is plain that whoever applies shampoo or the like to his hair will pour water over it afterwards to remove it. And usually water flows over it many times, such that some of the water would be changed a lot by the shampoo the first time [s. it flowed over it], then the water after that would be changed slightly [s. from the shampoo], and finally some of the last flows would be clear and unchanged. If this is the case, then the ghusl is valid and this is the usual case, plain and clear.
[Translated by Shazia Ahmad]