The Vatican said Tuesday that Catholics can receive transplants of animal tissues to address medical conditions, as procedures involving genetically modified pig or cow organs continue to advance.

In an 88-page document providing ethical guidelines for such transplants, the Vatican reaffirmed an earlier teaching and said the Church has no objection to such treatments, provided they follow best medical practices and do not treat animals with cruelty.

"Catholic theology does not have preclusions, on a religious or ritual basis, in using any animal as a source of organs, tissues or cells for transplantation to human beings," the document said.

The text addressed xenotransplantation, or the transplanting of organs or tissues from one species to another. The Vatican first approved such procedures in 2001, when they were in early stages of development. Animal organ transplants for human use are still rare. The first pig-to-human kidney transplant was carried out in the United States in 2024.

The Vatican document, which was drafted with the help of doctors from Italy, the U.S. and the Netherlands, called on scientists to pursue animal transplants in a manner that is "purposeful, proportionate and sustainable."

It also called on doctors to disclose the risks of animal transplants, including the probability of rejection by a patient's immune system and the possibility of causing infection from microorganisms.