If you are in Spain or any other spanish speaking country and you need to buy painkillers this is what you must say:

-Me da una (caja de) analgesicos por favor?
(Could you give me painkillers please?)
Notice that 'una caja' means a box.
or just use
Necesito un analgesico
(I need some painkillers)

or you can directly ask for the painkiller of your choice, names are similar: aspirina (aspirin), ibuprofeno (ibuprophen), paracetamol (paracetamol).