The right to refuse life-saving treatment: who decides?

District nurse Alison has a patient, Nigel, who has type 1 diabetes that remains unstable. Nigel's eyesight has deteriorated, his sexual function has dramatically reduced and he has now developed an infection in his right foot which requires immediate treatment. Nigel refuses the treatment and any further insulin declaring he would rather die than live the rest of his life like this. The GP says he will have Nigel detained under the Mental Health Act for assessment and enforced treatment, as his decision to refuse treatment amounts to suicide. Alison is concerned that Nigel will die without treatment, but also worries that to force treatment would be unlawful.