A Sydney teenager has lost his bid to refuse a potentially life-saving blood transfusion treatment because he is a Jehovah's Witness.

The 17-year-old cancer victim, known only as X, appealed against a NSW Supreme Court order that staff at the Sydney Children's Hospital should be allowed to give him the treatment.

He told his doctors he would rip the IV out of his arm if forced to have the transfusion, which he believes is akin to being raped.

X is suffering from Hodgkin's disease and his doctors believe he has an 80 per cent chance of dying from anaemia if he does not receive the treatment.

In March, Justice Ian Gzell ordered the hospital be allowed to carry out the treatment on the boy, who immediately appealed against the decision.

While noting he was highly intelligent, a "mature minor" and devoted to his faith, the judge also found the boy had been "cocooned in that faith" for his entire life.

"The sanctity of life in the end is a more powerful reason for me to make the orders than is respect for the dignity of the individual," Justice Gzell said at the time.

In a judgment handed down in the Court of Appeal today, the three appeal judges agreed.

"There is no reason why a different result should be achieved because the [boy] is now five months closer to his 18th birthday," the judgment said.

"The interest of the state is in keeping him alive until that time, after which he will be free to make his own decisions as to medical treatment."

The boy turns 18 on January 18 next year, when he will then be able to exercise his right as an adult to refuse further treatment.

AAP