In a huge achievement for regenerative medicine, a frog has regrown a severed leg after being treated with a cocktail of drugs.

An African clawed frog - a critter that does not naturally have this ability - was used to help rebuild an amputated limb. It worked after a shockingly short amount of medication exposure: 24 hours of treatment triggered an 18-month process of regrowth of a brand new, functional leg.

In a new study, scientists amputated a frog's hind leg and covered the incision with a silicone cap carrying a five-drug combination. Each of the medications served a particular objective, such as lowering inflammation and increasing collagen formation to prevent scar tissue from developing. The medications were also designed to stimulate the formation of new nerve fibers, blood vessels, and muscle.

The experiment was carried out on dozens of frogs, and many of them had significant tissue regrowth, with many of them re-creating a nearly entirely functional leg, complete with bone tissue and even toe-like features at the limb's end.

The frogs were able to swim using the regrown leg, which moved and responded to touch.

Salamanders, starfish, crabs, and lizards are among the species that can completely regrow at least some of their limbs. Flatworms can even be cut into pieces, with each piece reconstructing the whole organism.

Humans have some regenerative abilities; for example, after being halved, the liver can regenerate to full size, and children can regrow the tips of their fingers. However, no natural process in animals can repair the loss of a huge, complicated limb. Scar tissue forms quickly to protect us from blood loss and infection, but it also limits renewal.

Scientists observed the activation of molecular pathways that are ordinarily utilized to map out limbs in the developing embryo during the first few days of treatment. They believe that adult humans still retain the information required to build body structures and that, in theory, this dormant ability should be accessible.

"It's exciting to see that the drugs we selected were helping to create an almost complete limb," Nirosha Murugan, first author of the study, said. "The fact that it required only a brief exposure to the drugs to set in motion a months-long regeneration process suggests that frogs and perhaps other animals may have dormant regenerative capabilities that can be triggered into action."

The next step, according to the researchers, will be to improve the technique so that more full and functional limbs can be grown, as well as to see if the same process can be used in mammals.

The research was published in the journal Science Advances.