Morphine is a drug derived from Opium It is a heavy painkiller. Once sold over-the-counter, it is now strictly controlled, by the FDA, because of it's highly addictive properties.
Significance in Little House on the Prairie
Morphine's role in Little House is that of an addiction and as a means of using people.
Granville Whipple had been taking Morphine from a shrapnel wound in his leg that he suffered at Shiloh during the Civil War, but he also used it to numb the pain of the emotional scars over the lost of a friend during the battle. (Episode 221: Soldier's Return)
While Albert was living in Burr Oak, Iowa, he began taking Morphine because of peer pressure from his new friends. After it was discovered that Albert was an addict, Charles helped him through his horrible withdrawal symptoms which included violent spasms. (Episode 916: Home Again (Part 1), Episode 917: Home Again (Part 2))
Morphine was also given to Matthew Rogers addicting him to it and then withholding it from him to make him act out violently, for the purpose of making him into a circus attaction known as the "Wild Boy". (The Wild Boy (Part 1), The Wild Boy (Part 2))