Willenbring tested positive for the substance hydrochlorothiazide, a drug usually taken to treat high blood pressure or swelling due to heart failure or cirrhosis of the liver.

He served a four-month ban that has already expired.

It began on October 19, when his voluntary provisional suspension started.

As part of the suspension, Willenbring forfeited all results and medals from August 28, the last day of the World Junior Championships, when he helped the United States to victory in the men’s 4x100 metres medley relay.

USA Swimming has confirmed that the entire relay team will forfeit their medals.

Willenbring also won silver in the mixed 4x100m freestyle relay and bronze in the men’s 100m freestyle.

His attorney, Howard Jacobs, shared with SwimSwam the full FINA decision on his ban. 

A hearing was held before the FINA Doping Panel in Lausanne on January 12.

It was attended by Willenbring, his mother, Jacobs and a witness, Jackie Higgins.

Higgins, a former work colleague and close friend of Willenbring's mother, visited the family in August 2017.

She testified that she travelled with a supply of blood pressure medication, that included hydrochlorothiazide. 

It is claimed that when she travelled, she carried a supply of that medicine with her in a bottle of naproxen, which was left at the Willenbrings' house.

From August 14 through to August 20, Willenbring is said to have "several times needed an anti-inflammatory pill to address a bad headache". 

While looking for the family’s bottle of Ibuprofen, he supposedly found the bottle of Naproxen instead.

Willenbring recalled in his testimony that he used pain medication two or three times from August 14 to August 20, the week before departing for the World Junior Championships.

It was only when the family searched for the potential source of hydrochlorothiazide in late November 2017, that they learned of Higgins’ bottle of Naproxen being left at the house.

In its decision, the FINA Doping Panel concluded that it was not demonstrated "that the athlete acted with intent", which represents the first cut of the maximum four-year suspension for a first-time offence for a banned substance, down to two years.

The Doping Panel also said that because of Willenbring’s age, the requirements for no significant fault or negligence to further reduce his suspension from two years are not in effect.

But, owing to various factors, it believed the story of the mis-labelled bottle left by Higgins, leading to the further reduction of the suspension to four months.

FINA has also only just announced a doping ban for Serbian water polo player Filip Radojević, who tested positive for a banned substance last year.

Radojević is currently serving a two-year suspension from the sport after he tested positive for pseudoephedrine, a banned substance.

He tested positive on August 13, while in competition at the 2017 World Junior Championships in Serbia's capital Belgrade.  

Radojević's period of ineligibility begun more than two months later, on October 23.

FINA has previously delayed announcing doping offences.

Three-time Olympic champion swimmer Sun Yang of China served a secret three-month ban for doping in 2014 after a failed test for banned stimulant trimetazidine.

He had returned to competition before news of the suspension emerged.