Andrew's psychiatrist at Bellevue said a fellow patient, who later turned out to be a con artist faking a mental illness, convinced Andrew and other patients to trade medications. When someone is admitted to a psychiatric facility, or any hospital for that matter, they are not allowed to administer their own medication for obvious safety and liability reasons. Medication is always administered by a physician or a nurse and they always insure that the patient swallows their pills.
(At 20:13) When Fin receives an envelope from the woman who moved into Ken Randall's old apartment, the address reads 110 Central Park North. All address numbers along Central Park North are odd numbers (North side of CPN). Address number 110, an even number, would be on the south side which is Central Park side.