A drug used to prevent organ rejection in kidney transplant patients is being touted for potentially slowing down aging.

The immune-suppressing medication rapamycin, also known as sirolimus, is being used off-label as a means of extending longevity.

"By targeting a key pathway (mTOR) in cells, this drug has been shown to extend lifespan in animals and could be beneficial to prevent age-related diseases in humans," Dr. Andrea B. Maier — a professor in healthy aging and dementia research at the Director of the Centre for Healthy Longevity at the National University of Singapore — told Fox News Digital.

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Maier was co-author of a recent review study that explored how rapamycin affected healthy humans.

The review, which was published in the journal Lancet Healthy Longevity, found that the medication improved the effects of aging on the skin.

"Rapamycin and its derivatives improve the immune and cardiovascular function of healthy individuals or individuals with aging-related diseases," Maier said in an email.

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Daniel Tawfik, a molecular biologist and co-founder of Healthspan, a digital medical clinic that focuses on anti-aging and longevity-promoting therapies, said his team uses rapamycin to help slow down the rate of aging. 

"We view this through the lens of reducing the accumulation of damaged senescent cells," Tawfik, who is based in Santa Monica, California, told Fox News Digital.

Senescent cells are cells that no longer function properly, but linger in the body and can contribute to tissue deterioration and inflammation, the expert said.

"By lowering the rate at which healthy cells transform into senescent cells, rapamycin can slow the aging process at the tissue level, helping to preserve organ function and overall health as we age," he added.

In the U.S., rapamycin is approved by the Food and Drug Administration (FDA) for its immune-suppressing properties in patients with kidney transplants and certain cancers. 

It is not approved for use as an age-defying drug.  

"Sirolimus (rapamycin) has not been evaluated by the FDA for safety and effectiveness as an anti-aging treatment," a press officer for the FDA confirmed to Fox News Digital.

Despite this, rapamycin is being used "off-label" for anti-aging purposes, according to Elaena Quattrocchi, a registered pharmacist and an associate professor at Long Island University College of Pharmacy in Brooklyn, New York.

"Off-label means the FDA has not approved the drug for this use," she told Fox News Digital.

When prescribing a drug for off-label use, the provider must explain to the patient that the medication has not been approved for the indication for which they are prescribing it, Quattrocchi noted.

"People taking this medication must discuss with their primary care provider and pharmacist about the potential side effects and drug interactions," she said.

Tawfik’s company currently treats more than 3,500 patients with rapamycin, he told Fox News Digital. 

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His team has seen numerous benefits of its off-label anti-aging use, he said, including improved cellular health, reduced risk of age-related diseases and the potential for enhanced longevity.

"Under proper medical supervision, many of our patients experience a marked reduction in inflammation and improvement in conditions related to autoimmunity," Tawfik said.  

"For example, patients with osteoarthritis often report significant relief from joint pain after starting rapamycin."

The expert also noted that certain biomarkers linked to healthspan and longevity — such as reduced inflammation markers and enhanced metabolic health — were seen in the bloodwork of the patients using rapamycin.

Patients need a prescription for rapamycin, experts told Fox News Digital. 

Individuals are screened to determine whether they are candidates for the treatment. If it is prescribed, patients are monitored carefully when taking the drug.

"It is essential that rapamycin use is overseen by a physician to ensure that dosing is effective yet safe, as high doses can lead to unwanted immunosuppression," Tawfik warned. 

Maier echoed that rapamycin and its derivatives are not for self-medication use. 

"An important step is to first measure the patient’s biological age and investigate why the pace of aging might be faster," she told Fox News Digital. 

"Then a tailored approach to match the measures with interventions at need is required."

Quattrocchi and Tawfik both emphasized the importance of working with an accredited pharmacy that sources the drug directly from a reputable manufacturer.

Rapamycin is not recommended for immunocompromised individuals or those taking immunosuppressant medication, Tawfik cautioned.  

It is not typically used in younger adults, he added, since starting this regimen at such an early age is unnecessary.

There are limited studies on the effects of rapamycin on humans and organ systems, Maier noted.

In researching studies of the drug, Maier’s research team found that adverse reactions in healthy individuals were classified "as mild or moderate and were reversible after the discontinuation of treatment."

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However, the study authors stated, "there were increased numbers of infections and increases in total cholesterol, LDL cholesterol and triglycerides in individuals with aging-related diseases."

"Future studies should assess the remaining unexamined systems and test the effects of long-term exposure to rapamycin and its derivatives."

The majority of studies exploring rapamycin’s effects on aging have been conducted on mice and other animals as well as experimental models, researchers told Fox News Digital.

"The reason scientists are so interested in rapamycin is that in every species that’s been studied to date – yeast, worms, flies, mice – when they are given rapamycin, healthspan and lifespan are extended," Tawfik noted.

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"No other intervention has that degree of validation."

Quattrocchi agreed that further research is needed into the drug’s proper dosing, adverse effects, and potential anti-aging benefits in healthy individuals and those with medical conditions.