A debate is raging over whether the NHS should use its limited resources to fund complementary remedies.
Spinal manipulation is a complementary therapy
Leading doctors have there is no "scientific proof" that they work.
But what are complementary therapies, and why do they provoke such strong opinions?
What is the difference between alternative and conventional therapies?
Defining the difference is not straightforward.
But broadly, complementary - or alternative - therapies are those which are not proven to the clinical standards of Western medicine but which have been used, in some cases, for millennia to relieve people's symptoms.
The term complementary is often preferred, as the therapies are used alongside conventional treatments.
Practitioners say alternative is an inappropriate term, as the therapies are increasingly widely used.
Homoeopathy, acupuncture, reflexology, aromatherapy and a range of 'hands on healing' techniques such as reiki and shiatsu and herbal remedies are common forms of complementary medicine.
How are complementary therapies' benefits proven?
This is the nub of the medics' concerns.
For a medicine or procedure to be used in conventional medicine, it must go through scientific trials where its effectiveness has to be proven.
But, when complementary therapies are tested, conventional testing techniques often fail to show how they work.
Those, like the signatories of a letter to the Times on Tuesday, say this is because they do not.
But advocates of complementary medicines say it is because conventional testing methods cannot recreate the effect of the therapies reported by individuals.
Many medics, including the British Medical Association, want to see better regulation of the complementary therapies and whose who provide them.
How much research has there been into complementary therapies?
There have been a number of studies carried out, using the same methods as those used to test conventional medicines.
The majority do not show that complementary remedies have any extra benefit over a placebo or 'dummy' treatments.
But leading campaigners say there is considerable body of positive evidence that such therapies work.
And one study last year found acupuncture worked in its own right, and not because the person expected it to, as sceptics believe.
Is there any crossover between conventional and complementary medicine?
Yes. The Royal Homoeopathic Hospital is part of the NHS, and it is estimated that under half of family doctors provide some form of complementary therapy for their patients.
But the Patients Association has called for equal access, and said all GPs should be in a position to refer patients to a complementary therapist.