I found a good post in server fault on how we can speed up drop index in big table in MySQL. Here is the scripts.
CREATE TABLE patient_new LIKE patient; ALTER TABLE patient_new DROP INDEX post_text; ALTER TABLE patient_new DISABLE KEYS; INSERT INTO patient_new SELECT * FROM patient; ALTER TABLE patient_new ENABLE KEYS; ALTER TABLE patient RENAME patient_old; ALTER TABLE patient_new RENAME patient; DROP TABLE patient_old;