Conor McGregor Reportedly Kept Using Banned Drugs Post-Healing, UC Davis Prof Warns of Heart and Cancer Risks

Conor McGregor is alleged to have continued using banned drugs, including anabolic steroids and human growth hormone, long after his shattered leg had healed, despite a UC Davis professor warning such substances can raise the risk of serious heart conditions and cancer.

With the UFC star set to make his return to the octagon in Las Vegas this Saturday, two sources with knowledge of the matter claim the 37-year-old kept up the illicit regimen simply because he liked the energy boost it provided.

It can be recalled that the fighter suffered a lower leg break during a July 2021 match. In the aftermath of the injury, McGregor opted out of the official drug-testing pool for more than two years. Anti-doping experts argue this decision created a loophole, allowing him to use prohibited substances during his rehabilitation without the immediate threat of a sporting suspension.

The accounts offer a glimpse into the measures the athlete took to secure his return. McGregor previously insisted during a televised June interview that these steps were a medical necessity, stating he wanted to play with his children in a normal capacity again.

The bone specialist who prescribed the substances reportedly advised a short course lasting only weeks to aid tissue repair. Yet UFC officials allegedly learned from McGregor himself that he kept taking the drugs entirely for how they made him feel, beyond the original medical intent.

Heart And Cancer Fears Over Prolonged Drug Use

Taking such potent compounds for an extended period carries significant medical risks. Keith Baar, a professor of molecular exercise physiology at the University of California, Davis, warns these drugs carry severe long-term dangers.

Because it is widely considered unethical to subject human trials to the massive doses athletes consume, peer-reviewed data remains scarce. However, Baar notes the substances can significantly increase cardiovascular problems, cause organs like the heart to enlarge and stimulate the rapid growth of cancer cells.

The physical benefits, conversely, might last a lifetime. Rodent studies demonstrate that animals maintain an enhanced physical state long after the administration of these performance enhancers has stopped.

Anti-doping experts have expressed concern over the possibility that a competitor could retain an artificial biological advantage for years after concluding a cycle of performance-enhancing drugs. April Henning, an anti-doping expert at Heriot-Watt University in Scotland, described the situation as a 'nightmare for fair sport', noting McGregor essentially operated under a different set of rules.

Legal Questions Over HGH Prescriptions

Beyond the sporting arena, the administration of human growth hormone invites rigorous legal scrutiny. Under federal law, the substance is treated as a controlled pharmaceutical. The Food and Drug Administration permits its use only for specific conditions, such as major height deficiencies in children or wasting diseases linked to HIV.

Healing a fractured tibia is categorically not on that approved list.

Doctors caught prescribing human growth hormone for off-label purposes can face felony charges, though Lars Noah, a professor at the University of Florida Levin College of Law, points out such prosecutions remain rare. However, off-label prescriptions remain a regulatory concern.

Neal ElAttrache, the sports physician who oversaw the surgery, confirmed he recommended bone-healing specialists when he feared the fractures might not fuse.

While ElAttrache did not write the prescriptions, he wrote a letter endorsing McGregor's application for a therapeutic-use exemption to the United States Anti-Doping Agency. That exemption was never granted.

UFC Stands By McGregor's Comeback Bid

Despite the controversy, McGregor's management and the promotion remain firm in their defence. Audie Attar, the fighter's manager, released a statement insisting his client has remained in full compliance with the comprehensive drug programme. The UFC echoed this, stating the athlete sought appropriate medical guidance after sustaining a potentially career-ending injury.

The promotion also highlighted his recent testing record. After re-entering the testing pool in October 2023, McGregor was tested 22 times over a two-year span, providing 32 samples that all returned negative results. However, this record is complicated by his physical availability.

Consequently, the promotion suspended him in October 2025 for 18 months. Because the suspension was backdated to September 2024, he was officially cleared to compete in March 2026, paving the way for his scheduled comeback fight this month.

Whether any lingering biological effects of the anabolic steroids will give him an edge on Saturday remains an open debate, but the integrity of the bout is already under scrutiny.