“I trusted him because we were friends,” said Wasilisin. “The original agreement was that when they became successful, they’d buy me a boat—and the bigger the boat, the more I helped them sell.”

According to Wasilisin, he held up his end of the deal but claims he has yet to get the boat—or any compensation from Theragun sales, despite promoting the massage devices to his millions of followers on Instagram, YouTube, TikTok, and Facebook. In a lawsuit filed in California on Tuesday, Wasilisin accused Therabody of improperly leaving his name off two patents for the thumb and wedge tips that attach to the percussive massage devices. He’s seeking corrections to both patents to list him as the sole inventor, compensatory and punitive damages, and an accounting of all the Therabody products that have been sold with the attachments Wasilisin says he designed. David Hecht, a lawyer for Wasilisin, estimated the case could be in the multi millions. 

In a statement, Therabody general counsel Jonathan Feldman told Fortune the claims in the lawsuit are without merit.

“Therabody has a proud history of innovation and collaboration, and we take intellectual property rights very seriously,” Feldman said. “We intend to vigorously defend ourselves against these allegations and look forward to sharing the facts through the appropriate legal process.”

Therabody manufactures and sells a variety of fitness recovery and other devices, including its flagship Theragun. The high-end massager retails for $650, and includes a variety of tips to target different areas of the body and a multitude of muscle aches and pains. 

“At a high level, there’s a pool of revenue attributable to these attachments, and we’re seeking an appropriate portion given that these are Dr. Mike’s designs,” said lawyer Tanner Murphy, who is also representing Wasilisin. 

Therabody was founded by Wersland, who goes by Dr. Jason, after a 2007 motorcycle accident in Los Angeles in which Wersland T-boned a Volkswagen Jetta and went flying over the car. Wersland walked away from the accident without broken bones, but he suffered severe soft-tissue damage. To deal with the pain, he created the original Theragun using a Makita jigsaw and a dish towel. The company has since grown into a health and wellness empire, and raised hundreds of millions from private investors over the years. 

According to Wasilisin’s lawsuit, he reached out to Wersland in August 2017 and the two met up in Los Angeles. At that meeting, Wasilisin allegedly showed Wersland his thumb and wedge tip designs, prototypes, drawings and other documentation. He claims he agreed that Therabody could patent his designs, but only if he was listed as the inventor on patents and paid for his work. Two months later, he shipped the thumb and wedge attachments to Therabody’s offices in California. 

Wasilisin claims that five months after the LA meeting, Therabody filed two patents for the thumb and wedge massage tips—and didn’t inform him or list his name. Instead, Wersland’s name was listed on the patent along with two others. Wasilisin claims the three had nothing to do with his design and development.

In 2021, three years after the February 2018 patent filings, Wersland allegedly texted Wasilisin, “I will sign something for every idea you’ve give [sic] me. I will not take your ideas without you. You’re listed on the patent for the thumb. That’s how we do.”

However, it wasn’t until two years later, to his horror, Wasilisin looked up the patents and saw his name wasn’t listed. He immediately texted Wersland. 

“Dude, my heart just sank to the floor when I saw this. What happened?” he wrote, according to the lawsuit. 

“I don’t know,” Wersland wrote back, according to the suit. “I’ll find out. I will fix this.”

Wersland and the two others all signed inventor oaths, the suit states. The U.S. Patent and Trademark Office requires an oath or declaration statement confirming that an inventor, or joint inventor, believes they created or co-created what is being patented. A Therabody executive reached out to Wasilisin about one of the patents, but another year went by before Therabody offered to pay Wasilisin for the thumb attachment sales, he said. 

“I’d been promoting Theragun on all my channels for free, never received a penny in affiliate commission, got them hundreds of millions of views—and I did it because I believed in the product and trusted Jason,” Wasilisin told Fortune. “This is about lost friendship and lack of trust.”

Wasilisin believed that, eventually, his involvement would be formalized and he would be meaningfully compensated for his contributions. But he ultimately concluded he was never going to paid fairly without a fight, he said.

Wasilisin said Therabody offered him what he called a “bogus contract.” He ran the text through Grok AI and the analysis told him not to sign it by any means. He called Wersland and his former friend’s response utterly devastated him. He said Wersland allegedly told him, “Why can’t you just drop this?”

“Those words run through me like, he’s in the know that they’re trying to make me leave,” Wasilisin said. “This is somebody I trusted as a friend, and I loved this guy—still want to love him—and it’s so f—ing hurtful.”

Wasilisin said he wants to see Therabody—and his ex-friend Wersland—succeed but he also wants acknowledgement for his work, he said. 

“This is the rare case where I don’t think Therabody can dispute that they’re using his thing,” said Hecht.