All tested athletes have reportedly passed their drug tests following UFC 161. The Manitoba Combative Sports Commissioner has just released the following brief statement:
The Manitoba Combative Sports Commission had random drug testing conducted at UFC 161 in Winnipeg, Manitoba on June 15, 2013. Ten of the twenty two athletes were randomly tested for performance-enhancing drugs and recreational drugs/drugs of abuse. All ten athletes tested passed with negative test results. No therapeutic use exemptions were granted.
Thank you
Joel Fingard
Executive Director
Manitoba Combative Sports Commission
UPDATE – July 18, 2013 – Today Bloody Elbow reports that Dan Henderson did not even apply for a Therapeutic Use Exemption for Testosterone Replacement Therapy due to the Manitoba Commission’s requirements for this. You can click here to access my previous article discussing these strict and sensible requirements.
Combat Sports Law 
Why not all of them? Why just 10 fighters?
Reggie, thanks for your comment.
The Manitoba Law allows for “random” drug testing. Manitoba’s Boxing Act allows regulations to be passed “providing for the drug and alcohol testing of professional boxers on a random basis or otherwise”
Section 48 of the Regulations was passed which reads as follows “The commission may require a boxer to undergo random drug testing
for performance enhancement or illicit drugs. When required to undergo such
testing, a boxer shall report for and undergo the testing at the time and place
indicated by the commission”
Another question that was posed on Twitter was “which 10 fighters”. Manitoba Privacy laws likely prevent the release of such information. Section 49 of the regulation reads as follows:
“Except for the purpose of enforcing the Act and this regulation, the
commission and its members, employees and agents shall maintain confidentiality
with respect to any medical report, medical certificate, and any related medical
information in its or their possession.”
I’ve discussed this further here: http://canadianmmalawblog.com/2013/06/28/why-the-manitoba-commissions-silence-is-golden-when-it-comes-to-fighter-injury-details/