Researchers have observed some adverse health effects in sheep and other mammals that have been cloned. These include an increase in birth size and a variety of defects in vital organs, such as the liver, brain and heart. Other consequences include premature aging and problems with the immune system.

Why is cloning not safe?

They found that cloned mice that were healthy in all outward respects carried a high "burden" of genetic abnormalities, which could shorten their lives. The cloning process has also been shown to cause a higher-than-normal incidence of birth defects. ... "It seems very unwise to attempt this sort of cloning on humans."

Why is human cloning banned?

In addition to the above ethical considerations, research cloning should be forbidden because it increases the likelihood of reproductive cloning. Preventing the implantation and subsequent birth of cloned embryos once they are available in the laboratory will prove to be impossible.