A few friends have asked me the same question.
They are currently in ROTC, and lied about past drug usage in applying for a secret security clearance.
Now, later in their career, some are considering becoming intelligence officers. They are scared that admitting to lying about the drug usage in the past will not only disqualify them from obtaining a TS/SCI, but may also get them kicked out of ROTC/clearance revoked.
If they came clean about past drug usage AND past lying on the secret clearance application, would this be an automatic disqualification from getting a TS/SCI? Would it get their secret clearance revoked?
One friend did drugs while he had a clearance, the other friend stopped drugs after he got his clearance, but still lied on his application.