Trainee Detective Constable Laura Stanley, 39, was arrested in August and asked to voluntarily provide a sample of hair for analysis.
A subsequent test showed that between June 1 and August 31 she had ingested the controlled drug cocaine.
She was later formally cautioned for offences under Section 55 of the Data Protection Act and Section 40D of the Prisons Act 1952.
At a public gross misconduct hearing held at Sussex Police HQ on Friday (December 2), it was found that she had breached the standard of professional behaviour relating to discreditable conduct and she was dismissed without notice.
Detective Chief Inspector Nick Wainwright, from Sussex Police’s Professional Standards Department, said: “Police officers must behave in a manner which does not discredit the police service or undermine public confidence, whether on or off duty.
“Clearly, this officer’s conduct, compounded by her admission that she had taken cocaine, is completely out of line with the standards and professional integrity that we expect from our officers and staff.”