Ordinarily I would say that the discounts are subject to the same rate of VAT as the original sales invoice
However, I am aware of discounts being given by large pharmaceutical companies which are at the 0% rate of VAT (even if part of sale is at 21%) and Revenue don't seem to be challenging it, perhaps because the VAT is reclaimable by the pharmacy anyway
If the customer is able to reclaim VAT I would err on the side of caution and issue the discount with VAT.