Junior doctors in Wales are striking for three days from 7am on Wednesday.
The strike is over pay and working conditions, with 98 per cent of British Medical Association (BMA) in Wales members in favour of strike action.
Judith Paget, the head of NHS Wales, has urged people to help reduce the burden on the NHS by considering other options besides hospital, unless they need urgent care.
Paget said: "As during the last strike we welcome the collaborative approach on patient safety taken by the British Medical Association. We have been working with them and NHS organisations to ensure patient safety is protected during the industrial action.
"We expect significant disruption to non-emergency and elective activities during the industrial action, with much work already been postponed. During the last strike around 41 per cent of outpatient appointments and 61 per cent of operations were postponed across Wales. We expected services during the industrial action to resemble those generally provided on a Bank Holiday.
"However, if you have a critical need to attend an emergency department you should still do so.
"But we are also urging everyone to consider the best option for them, including 111 online or phone service, or their local pharmacy."
Welsh health minister Eluned Morgan has said that the Welsh Government are open to further talks, but they don't have the budget to increase the pay deal.
Morgan said: "We are disappointed that junior doctors have decided to take further industrial action in Wales, but we understand their strength of feeling about our 5 per cent pay offer.
"Our offer is at the limits of the finances available to us and reflects the position reached with the other health unions. But we will continue to press the UK Government to pass on the funding necessary for full and fair pay rises for public sector workers.
"We remain committed to working in social partnership with the British Medical Association and are always open to have more talks."
On Friday, director of the Welsh NHS Confederation Darren Hughes said: “NHS leaders understand the financial challenges facing the Welsh Government, but it is disappointing that the BMA and government have not been able to find a solution to the ongoing pay dispute.
“Patient safety is of course the number one priority. Health leaders and their teams will pull out all the stops to fill rotas ahead of the strikes, but the reality is more patients having their operations or appointments cancelled, which will impact on attempts to tackle waiting lists for routine hospital care.
“As always, it is patients who bear the greatest burden from industrial action, with around 41 per cent of outpatient appointments and 61 per cent of operations postponed across Wales in the last wave of industrial action by junior doctors in January.
“The impact of strike action is not just felt on the days of industrial action, but also following industrial action when the service does its best to catch up on postponed patient appointments and cancellations.
“It is not too late to restart negotiations and stop the strikes before more patients have their vital care delayed. We call on both sides to lay out the basis for a realistic settlement that would be satisfactory to the majority of junior doctors.”
