New Zealand lock Patrick Tuipulotu has been cleared of doping and is free to return to rugby immediately.

A doping control B sample from the 24 year-old tested by a World Anti-Doping Agency (WADA)-accredited lab in the United States showed no presence of a banned substance.

Tuipulotu's provisional suspension has been lifted and World Rugby will not be taking the matter any further.

"This has been a challenging and difficult time for me, my partner, family and team-mates, and I am grateful for their support," the Blues lock said. "I'm pleased the matter is now closed, with my reputation and integrity intact, and I look forward to getting back into training and an exciting year of rugby."

New Zealand Rugby Players' Association chief executive Rob Nichol said the Sports Medicine Research and Testing Laboratory (SMRTL) was investigating the reason for the difference in A and B samples.

"This is a matter that SMRTL is investigating and we look forward to their feedback," he said. "This result also reinforces the importance of the regulations and strict confidentiality obligations regarding players and their rights.

"Patrick was unfairly labelled as a result of the premature publicity of this matter. We trust that everyone now has a greater level of understanding in this regard."