Why is there a black market for Morphine Sulfate Concentrated Oral Solution 20mg/1mL?

My friend's father died recently. Before he ped away, he was prescribed "Morphine Sulfate Concentrated Oral Solution 20mg/1mL." My friend is kinda f'ed up and says that there is a "black market" for this stuff. I guess it makes people feel good, but it is actually a pain reliever I think. Anyway I was afraid, so I spilled it down the toilet. But what is this used for?