Biohacker Bryan Johnson is taking criticism of his $2-million-a-year age-reversing regimen on the chin—even if it comes from X owner Elon Musk.

Centimillionaire Johnson may now be used to public scrutiny of his lifestyle: which includes eating his last meal of the day at 11 a.m., following a strict health “protocol,” and working with a team of 30 scientists to reverse his biological age from 46 to 18.

However, critics of Johnson have recently added a loud voice to their ranks: Tesla CEO, and the world’s richest man, Musk.

On Dec. 29 a user on X compared two images of Johnson before and after he began his age-reversing process. “Bryan Johnson looked way better before he started spending $2 million/year on his body,” the caption read.

Musk agreed, replying “100” to the post, and has now amassed more than 1,400 likes on his response.

Johnson quipped back to the thread, writing: “Former Bryan didn’t get invited to vampire cosplay as himself.” Cosplaying is when a person dresses up as a character from popular culture, with Johnson suggesting he can now pass as an ageless vampire thanks to his regimen.

On Dec. 31, Johnson, who carries out his research via his company Blueprint, screenshot the thread with Musk and wrote: “The difference between Elon and me: I’ll nourish you and drink your blood; he’ll fire you and leave you to die.”

This comment may be another tongue-in-cheek reference to the fact that Johnson is reportedly aging at a different pace from other people, similar to vampire characters in fictional books.

Johnson’s response also alludes to the backlash Musk faced in 2023 around his actions at X.

Since taking over the platform in 2022 Musk has shaved the business’s headcount by around 80%, saying the process of redundancies was “painful” but necessary to make the platform profitable.

However, that may still not be enough. Just this week Fidelity released a report claiming X is now worth 72% less than its value when Musk purchased it, which was $44 billion in April 2022.

The Tesla and SpaceX CEO, who has recently been focused on the launch of his xAI business and bot Grok, has also been open about X’s chances of survival.

Late last year Musk told DealBook Summit the platform may not survive and placed the blame squarely at the feet of advertisers who abandoned the platform.

Johnson’s rise to fame

Despite launching Blueprint some years prior, 2023 was the year Johnson’s work increasingly garnered the attention of global audiences.

The tech entrepreneur had been a relatively unknown individual until then, having sold his company Braintree for $800 million to PayPal.

Over the past 12 months however, Johnson was featured at Fortune conferences, profiled by the New York Times and Time magazine among others, and made numerous appearances on the Diary of a CEO podcast.

2023 also marked the year a woman became a Blueprint subject. Last summer Kate Tolo, who is referred to as Blueprint XX, began her trial of the regimen, saying she and Johnson are now “two peas in a pod.”

It’s perhaps no surprise that Johnson is looking into 2024 with wide-spanning goals in mind.

Posting on X, Johnson wrote his aims for this year include building his “Don’t die” community, open-sourcing his research into slowing aging, and making his nutrition program cost-competitive with the launch of fast food.