In Britain adults have the right to consent to experimental therapies with little chance of success, just as they may refuse treatment.
If doctors do not deem a treatment to be in a patient's interests they may refuse to give it, but a court cannot prevent an adult from seeking (and paying for) such care elsewhere.
In the cases of children or others who lack the capacity to decide for themselves and have not previously made their wishes clear, courts can overrule the wishes of families.
In America courts have been reluctant to go against parents' requests for treatment, even when the chance of success is vanishingly small, says Dominic Wilkinson of Oxford University.
That can mean doctors having to provide care which they think is against the interests of their patient.
Under Texas's “futile care law”, if medics feel that a treatment requested by a family will be of no benefit they can go before an ethics committee.
If the committee agrees that the treatment is futile, the family may seek another doctor to provide it, within a time limit.
In the Canadian province of Ontario a tribunal hears cases where doctors challenge families' decisions.
It often rules in favour of clinicians.
Disagreements between parents and doctors over end-of-life decisions involving children are uncommon.
One Dutch study suggested that they occur in about 10% of cases involving severely ill babies.
Even then, in all those instances, consensus was eventually reached.
Cases that reach a court are the exceptions.
They are, by their nature, the hardest of all.