The 23-year-old American tested positive for heart drug trimetazidine after an out-of-competition test on February 5 in Austin in Texas.

She was banned by the International Swimming Federation's (FINA) Doping Panel before she launched an appeal.

FINA then accepted that the swimmer had not taken the substance deliberately, but said they could not reduce the ban as the source of the trimetazidine had not been determined.

Cox, who had concluded the contamination could only have come from tap water, then decided to test a supplement, Cooper Complete Elite Athletic multivitamin. 

The American has taken this for seven years and not failed a drug test, but traces of trimetazidine were detected during testing in Salt Lake City.

Both FINA and the United States Anti-Doping Agency (USADA) assisted with the analysis.

After accepting this evidence, CAS has slashed the ban to six months.

As her initial ban started on March 2 she will be able to return to swimming from tomorrow.

"The last six months have been a grave and harrowing learning experience that I would not wish on any honest, clean, elite athlete," she said. 

"I know that any supplement - even a multivitamin purported to contain only those ingredients specified on the label and purchased at a local supermarket - can be suspect.

"I would advise any athlete who chooses to take supplements to not only consult resources such as USADA’s Supplement 411, but to also make sure that they are third party batch tested to check for any contamination.

"I am excited to return to the pool. 

"I also appreciate the support so many people have shown me throughout this trying ordeal."

Cox won World Championship gold in the 4x200 metres freestyle relay at last year's World Championship in Budapest.

She also won 200m medley bronze in the Hungarian capital.