Erika writes Jack out a prescription for Clonazepam. ("Something Nice Back Home")
Chemical structure of Clonazepam.
Benzodiazepines are tranquillizers and sleeping pills used therapeutically to produce sedation, induce sleep, relieve anxiety, muscle spasms, and to prevent seizures. Too much, used too often, can lead to memory loss. If used for extended periods, they can cause dependency and depression. The side effects of clonazepam are worsened when taken with alcohol.
On the Island, Hurley's sanity was challenged. Despite his use of humor, he struggled with stress; Sawyer called him "crazy" among other demeaning things, which he resented, and he continued to battle his eating disorder as the food drop tempted him. At one point, Hurley asked Sawyer if he had clonazepam in his stash because his old friend Dave reappeared and tormented him. ("Dave")
Off the island, Jack was plagued by visions of his father Christian Shephard. Believing this to be a hallucination, he asked Erika Stevenson to write him a prescription for clonazepam and she did so, but advised that he seek psychiatric help for his problems. ("Something Nice Back Home")
Upon receiving his prescription, Jack apparently ingested his clonazepam together with alcohol, which is known to intensify the drug's effects. ("Something Nice Back Home")