The NHG now requires a senior doctor to countersign any changes in the dosage of some toxic medicines, like the heart drug digoxin and blood thinner warfarin.

An electronic prescription system has also been in place since last year that will alert doctors to a patient's drug allergies and if the prescription is likely to interact with other medicine the patient is taking

Although the coroner's inquest this week returned an open verdict on the cause of Madam Koh Ah Tow's death, it found that the Clementi Polyclinic doctor had given her a dose of heart medicine, digoxin, that was four times higher than what she should have been given.

Expert evidence found that it could have contributed to her death. The doctor had claimed that she had called the pharmacy to change the dose, but the two pharmacy technicians on duty denied receiving such a call.

A spokesman for NHG, which runs Clementi Polyclinic, said doctors who want to change a prescription must now amend and reprint it.

SingHealth, the other public health cluster, also has an electronic prescription system at its polyclinics. Its spokesman said there are usually two to three changes in prescriptions at each polyclinic every day.

Doctors and pharmacists The Straits Times spoke to said errors in prescriptions do happen, but only rarely. They also said it is extremely rare for a patient to go off with an incorrect dose, because qualified pharmacists provide an important safety check.

A former hospital pharmacist said: 'That's what I spent my life doing, making sure that doctors have not made a mistake.' He added that he frequently picked up mistakes in prescriptions.

Colorectal surgeon Francis Seow-Choen, who is now in private practice, agreed that doctors do make mistakes. But the system has been set up to prevent a patient from walking off with the wrong medicine.

Dr Daphne Khoo, head of endocrinology at Singapore General Hospital, said: 'Almost every day, I get a call from the pharmacy to check on a prescription I've given.

'Most of the time, it is correct. But occasionally they do spot mistakes.'

When the dosage given to a patient is changed, the dispensing pharmacist asks the patient if the doctor has told him about the change.

If the answer is no, he checks with the doctor, she said. Even if the answer is yes, the pharmacist will call the doctor if he thinks the dose may be wrong.

Dr Khoo added that both doctors and pharmacists are particularly careful with medicines like digoxin and warfarin, where a slight change in dose could be crucial.

Dr Terrance Chua, head of cardiology at the National Heart Centre, said the normal range of digoxin is between 0.0625 and 0.25mg.

Asians normally get lower dosages, he added.