Drop down menu/ item grouping to be added to choose a specific item while PR

while configuring the ERP for a healthcare client, we need to make items/ medicines as items with different strength (mg), salt or different generics. How to add/ modify different dropdowns for such item configurations to be mapped with the item and this to be required while creating a purchase requisition as well?

for eg: while raising a PR for " paracetamol", we need to require a dropdown menu to take the input for the strength (mg) for the medicine.