Hello and welcome to Healthcare Magic. Thanks for your query.
Hair tests for drug screening are very sensitive and accurate. Like you have mentioned rightly, hair tests can pick up drug use even several months ago.
Secondly, such tests are specific for specific substances. So a test for codeine
would detect codeine specifically. Codeine can be found in certain cough
syrups and certain pain killer or sleeping aid preparations. So, if you have had a positive hair test for codeine, it's almost certain that you have taken codeine in the recent past.
Wish you all the best.
Dr. Jonas Sundarakumar