Crown Princess Mette Marit of Norway has started treatment for pulmonary fibrosis, a chronic lung condition that causes the lung tissue to become damaged.
The Norwegian Royal House confirmed on Friday that the Crown Princess, 51, will start her treatment immediately.
"Previous experience has shown that the medicines can lead to side effects that can have consequences for the Crown Princess's official programme," the Royal House said (translated to English).
"Therefore, we find it natural to share this information this time. We will inform you about changes to the official program if necessary."
It was revealed in 2018 that the Crown Princess had been diagnosed with a chronic pulmonary disease.
At the time, Mette-Marit said: "For a number of years, I have had health challenges on a regular basis, and now we know more about what is involved. The condition means that my working capacity will vary.
"The Crown Prince and I are choosing to make this public now partly because in future there will be a need to plan periods of time without an official programme to accommodate treatment and when the disease is more active."
The disease is incurable and progressively worsens over time, but the royal's doctor, Professor Kristian Bjøro at the National Hospital, has been monitoring the progression of the disease over the years and says: "The development of the disease over this period has been slow."
He is also confident that collaboration with medical teams abroad and the Crown Princess's participation in treatment trials will help with the disease.
What is pulmonary fibrosis?
According to the NHS, Idiopathic pulmonary fibrosis (IPF) is a serious condition in which the lungs become scarred and breathing becomes increasingly difficult.
It's not clear what causes it, but it usually affects people who are around 70 to 75 years old, and is rare in people under 50.
Symptoms can include a range of respiratory difficulties including shortness of breath and a dry cough. Patients can also experience extreme fatigue, unintentional weight loss, painful muscles and joints and a widening and rounding of the tips of the fingers or toes, called clubbing.
Several treatments can help reduce the rate at which IPF gets worse, but there's currently no treatment that can stop or reverse the scarring of the lungs. In some cases, patients will need a lung transplant.