Final guidance to doctors on the prescribing of Viagra and all other impotence treatments comes into force.

Pfizer, the company which manufactures Viagra, says it is still considering a legal challenge to the restrictions, following a High Court ruling which branded earlier guidance on the drug "unlawful".

From now onwards, doctors will be able to write private Viagra prescriptions for their patients.

Frank Dobson is worried about high demand for Viagra

Patients who were receiving treatment for impotence from the NHS on or before 14 September last year will get treatment.

If not, only men who have been treated for prostate cancer, who have had polio, have spina bifida, Parkinson's disease, or have severe pelvic injury will qualify.

Also included are men who have been treated for kidney failure by transplantation and dialysis.

And men in "severe distress" because of their condition, but who do not otherwise qualify can get treatment after consultation by a hospital specialist.

Fears about rampant demand

Health Secretary Frank Dobson introduced the restrictions, which will mean only 17% of those who could benefit get Viagra or other impotence treatment, because he feared the NHS would be swamped by demands for the new drug.

Some managers had warned that widespread access to the drug would cost �100m a year to the NHS.

Mr Dobson said GPs should restrict their prescribing to one pill per week. Each pill costs �4.86 to the NHS.

Originally, in January, he ordered GPs to restrict prescribing even further until a consultation process on the issue had ended.

Doctors' leaders such as the British Medical Association (BMA) condemned this as "irrational" and advised members to disobey the advice, saying they had an obligation to treat patients with necessary treatments.

The BMA had cautiously welcomed the slight relaxation of the restrictions, adding that some men with a definite clincial need for Viagra would still be "arbitrarily" denied it.