This a clinical study to improve medication adherence among patients with hypertension in Switzerland. Adult men and women who suffer from high blood pressure and have been prescribed a therapy consisting of at least 4 tablets per day can participate in this study.

The investigators intend to examine whether Collabree, a mobile phone application, is effective in supporting patients with hypertension to more successfully following their therapy plan. The study will also investigate if the use of Collabree can help lower blood pressure.

The study consists of 4 visits that take place during a 90-day adherence promotion program plus a 90-day follow-up period. There is also a screening visit before the study to determine whether the participants qualify for the study. The visits are carried out at the University Hospital Basel. Each visit lasts about 1-2 hours. During the visits, blood pressure is measured in the clinic and also through 24-hour ambulatory blood pressure monitoring (ABPM). Participants will also fill out questionnaires.

Participants are randomly assigned to one of 3 groups in a ratio of 1: 1: 1. Two of these groups will receive the Collabree mobile phone application. All participants will receive a box for storing their antihypertensive medication. This box serves as a system for measuring medication intake as it records the time the box is opened and closed. Standard care will given to all participants.

Inclusion Criteria:

1. Informed consent as documented by signature
2. Over 18 years of age at date of randomization
3. Primary or secondary arterial hypertension
4. Patient in an outpatient clinical setting
5. Prescribed a therapy consisting of 4 or more tablets taken per day
6. Stable medication regime that patient has been taking for at least 4 weeks
7. Participant administers their own medications
8. Participant owns a smartphone with a data plan or constant internet access during the study visits and at home to use the application
9. Minimal required smartphone operating system is iOS 12 and Android 8 (Oreo)
10. Adequate communication in German or Swiss-German (all study documentation will only be set to the German language).

Exclusion Criteria:

1. Cognitive impairment that limits ability to understand and complete questionnaires
2. Ongoing evaluation for secondary forms of hypertension
3. 3. Changes in the participant's medication treatment plan after the baseline visit during the intervention period, unless due to a diagnosis of symptomatic hypotension (in-clinic blood pressure below 110/60 mmHg and symptoms of orthostasis or dizziness) or stage 2 hypertension (in-clinic blood pressure > 160/100 mmHg); after the endpoint (day 90) visit, a change in the treatment plan will be permitted
4. Uncontrolled hypertension (in-clinic blood pressure > 180/110 mmHg)
5. Inability to operate a mobile phone and the Collabree application
6. Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc.
7. Pregnancy or intention to become pregnant in next 6 months
8. Enrollment of the Sponsor or Investigator, their family members, employees and other dependent persons
9. Participation in another clinical trial
10. Physician-estimated life expectancy of less than 6 months