The goal of this randomized intrapatient cross-over study is to assess the efficacy of oxybutynin versus venlafaxine in reducing hot flashes in women using endocrine therapy after breast cancer.

The objectives it aims to answer are:

• To assess the efficacy of oxybutynin versus venlafaxine in reducing hot flashes in women using endocrine therapy after breast cancer
• To assess side effects of oxybutynin versus venlafaxine.
• To assess the personal preference of women for oxybutynin versus venlafaxine in reducing hot flashes.
• To assess quality of life of women when reducing hot flashes in women using endocrine therapy after breast cancer.

Participants will fill-out a patient diary during 15 weeks total on a daily basis and receive an (online) questionnaire three times total.

Researchers will compare two groups (venlafaxine group versus oxubutynine group) to assess its efficacy concerning hot flashes.

Inclusion Criteria:

• Pre-, peri- or postmenopausal women of 18 years or above;
• Indication for endocrine therapy and already started with tamoxifen, aromatase inhibitors or luteinizing hormone-releasing hormone analogues for at least 4 weeks and planning to continue for the duration of the study;
• Experiencing hot flashes with a minimum of 14 per week for at least 1 month and desire to start a pharmacologic intervention.

Exclusion Criteria:

• Pregnant;
• Breast feeding;
• Patients who receive chemotherapy or immunotherapy/HER2 antibodies within the prior 8 weeks, and patients scheduled for chemotherapy during the study period;
• Palliative setting;
• Use of venlafaxine or any other antidepressants, also including St. John's wort within the previous year;
• Creatinine clearance < 30 ml/min;
• Liver cirrhosis;
• Use of gabapentin and/or calcium channel antagonists within 2 weeks of study entry;
• Use of oxybutynin before study entry;
• Use of any other substances or therapies for the treatment of hot flashes, for instance acupuncture.

Study protocol, anonymised database and study results may be available on request, after assessment of the reason for request by the study investigators.