Former world champion Parker produced a positive sample during a drugs test conducted on the day he faced 11th-round defeat against British heavyweight Fabio Wardley.
Wardley and the New Zealander engaged in a thrilling contest in London on 25 October to decide who would become next in line for the WBO to unified heavyweight titleholder Ukrainian champion Usyk.
Joseph Parker had positioned himself as next in line to face Usyk following putting together an impressive six wins in a row, yet put his mandatory challenger status on the line during the bout and suffered a stoppage in round eleven.
An ordinary examination administered to Joseph Parker through the Voluntary Anti-Doping Agency on the day of the bout showed an adverse result which leaves Parker in a difficult position to demonstrate his cleanliness.
The boxer's representatives published a statement which read: "The anti-doping body notified all required parties about Parker tested positive after a drug screening conducted on the fight day. While the matter is investigated, no additional comment are planned currently."
Parker earned the title of titleholder in 2016 until he lost his WBO belt by unanimous decision against Anthony Joshua in Cardiff 24 months later.
The fighter became a frequent visitor across the United Kingdom after that, competing in eight bouts altogether in the UK and relocating to Dublin for fight camps under highly regarded trainer former champion Lee during this period.
Under the tutelage of Lee, the boxer bounced back from the 2022 setback versus Joe Joyce to achieve a string of victories positioning him as the top contender to challenge the champion until he lost against Wardley.
After the boxer produced an adverse finding during the Wardley bout, allegedly for cocaine, an investigation about the test outcome is underway.