Shannon's inhaler contained her asthma medication. Boone packed 4 refills in his checked bag. However, after the crash the refills could not be found. The discarded empty inhaler was later discovered by Jack and Hurley when Jack stepped on it near the caves.
Jack finding Shannon's inhaler.
Shannon suffered from asthma since she was a child. Because she tended to forget her medicine, Boone had her inhaler on the plane and packed 4 refills in his checked luggage. While in their seats, Shannon searched her bag for her inhaler. Boone smiled and handed the inhaler to her. ("Exodus, Part 3")
Shannon continued to experience asthma attacks on the Island, even though some of the other survivors' physical ailments ceased after the crash. She hid her attacks from the others by sneaking hits from her inhaler in private. After her inhaler had run out of medication, an attack at the caves became life-threatening. Boone could not find the refills that he had packed in his checked luggage and began to suspect that Sawyer, who was reading Boone's copy of Watership Down which was also packed with the refills, was hoarding them with other items he confiscated from the plane. Sawyer refused to cooperate with the investigation, which led to him being tortured by Sayid. It was later discovered that Sawyer did not, in fact, have the refills. Shannon's attack was soothed by Sun, using eucalyptus leaves. ("Confidence Man") Shannon apparently suffered no further attacks between Day 9 and her death on Day 48.