USA Swimming organization just levied a 3 month suspension on Phelps, he cannot participate in or represent the United states in any competition during the suspension period.
In addition to the suspension, Kellogg’s corporation, which has featured Phelps on its Frosted Flakes and Corn Flakes cereal boxes, has cancelled Phelp’s endorsement sponsorship agreement, the company says Phelps “ does not fit the image that they want to portray”. “Michael’s most recent behavior is not consistent with the image of Kellogg,”
USA SWIMMING says,
In May 1998, Gary Hall Jr, who won four swimming medals at the 1996 Olympics, was suspended by swimming's international governing body for testing positive for marijuana. Hall came back to win six medals at the 2000 and 2004 Olympics, but Hall said that his failed drug test caused him to lose all of his endorsement deals. Swimming manufacturer Speedo cancelled Gary Hall’s endorsement sponsorship contracts following his drug suspension.
Phelps can advertise for cannabis clinics if he wants, but he wont receive a fraction of the money he would from mainstream sponsors, and he will lose those mainstream sponsors and the right to represent the United States in athletic competition with subsequent violations of USA Swimming and Olympic Committee rules.
But he’s an adult, and as such, he can do as he chooses.