B767 CREW HAD AN ERRATICALLY BEHAVING PAX ENRTE OVER THE NORTH ATLANTIC.

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ERRATIC BEHAVIOR OF PAX. EARLY IN OUR XING; THE PURSER NOTIFIED ME OF A PAX ACTING ERRATICALLY. HE HAD MADE MULTIPLE TRIPS TO THE LAVATORY RETURNING TO DIFFERENT SEATS IN THE CABIN AND WAS LOOKING VERY NERVOUS. WHEN CONFRONTED; HE WAS VERY EVASIVE. SUSPECTING A POSSIBLE 'SHOE BOMBER' TYPE; I MADE CONTACT WITH DISPATCH TO FIND OUT ANY ADDITIONAL INFO AVAILABLE. HIS MOTHER HAD TRIED TO CANCEL HIS TICKET FEARING A 'POSSIBILITY OF A SEIZURE.' WITH THAT INFO; I REQUESTED DISPATCH CONTACT THE MOTHER TO EXPAND ON HER PREVIOUS STATEMENT. DISPATCHER WAS VERY HELPFUL IN PRESSING MY POINT AFTER INITIALLY BEING TURNED DOWN BY MGMNT. THE FLT ATTENDANTS WERE ALSO GREAT THROUGHOUT BY KEEPING AN EYE OF HIM -- BEING PREPARED FOR THE WORST AND COMMUNICATING WITH ME THROUGHOUT. THE FINAL WORD WE RECEIVED ON HIM WAS THAT HE WAS SEEKING A MENTAL HEALTH PROFESSIONAL IN THE STATES AND HIS ERRATIC BEHAVIOR WAS POSSIBLY DRUG RELATED. WE SHOULD HAVE BEEN ADVISED OF HIS POTENTIAL BEFORE WE LEFT THE GATE. THIS MINDSET HAS GOT TO CHANGE BEFORE SOMETHING VERY SERIOUS HAPPENS THAT COULD HAVE BEEN PREVENTED WITH THE PROPER INFO IN A TIMELY MANNER.