More than 70% of patients with cancer in the palliative phase have pain that often requires treatment with opioids (morphine-like agents). Constipation occurs in 59% of patients treated with opioids. Opioid-induced constipation (OIC) has consequences that range from daily discomfort with social insecurity and disability to intestinal obstruction. It leads to limitations in self-management, a reduced quality of life and a risk of the need for more care. In the guideline "Diagnosis and treatment of pain in patients with cancer" it is recommended to start preventively with an osmotic laxative such as macrogol/electrolytes or magnesium hydroxide when starting opioids. Macrogol/electrolytes has been proven to be effective for OIC, but is sometimes perceived by patients as unpleasant due to its taste. Magnesium hydroxide, which is less commonly prescribed for OIC, has a neutral taste. Although it is mentioned in the guideline, it is not studied for the treatment of OIC and also not officially registered for this. To support the advice of the guideline and to prove that a choice is possible, it is important to investigate whether there are differences in effectiveness and/or side effects between macrogol/electrolytes and magnesium hydroxide in the prevention of OIC.

The aim of this study is to compare macrogol/electrolytes with magnesium hydroxide in the prevention of opioid-induced constipation in patients with cancer in the palliative (incurable) phase, who start opioids because of pain. The choice of laxative is determined by drawing lots (randomisation). After two weeks, its effect will be assessed and will be presented as the percentage of patients who have not developed constipation after starting opioids. If the laxative, as assigned by lot, is described as satisfactory by the patient, the patient can continue with the drug that the patient used, after the study through regular prescription.

The investigators will ask the patients in the study about their satisfaction with the laxative used, any side effects and the degree of pain. Furthermore, the appeal to care for possible constipation will be examined. Because the best attainable quality of life in the palliative phase is the objective of all care, the investigators will also measure this perceived quality with a questionnaire in this study. The results of this study will lead to the best achievable prevention of opioid constipation in patients with cancer in the palliative phase.

Inclusion criteria:

In order to be eligible to participate in this study, a subject must meet all of the following criteria:

• Patients with metastatic cancer (≥18 years);
• Starting with slow release or transdermal opioids for pain;
• Able to complete a Dutch questionnaire. Previous treatment with opioids is allowed, if discontinued more than 4 weeks ago.

Exclusion Criteria:

A potential subject who meets any of the following criteria will be excluded from participation in this study:

• Patients with contra-indications for laxatives
• Use of laxatives during the last four weeks
• Severely impaired renal function (serum creatinine >180 umol/l)
• Estimated life expectancy <3 months