The goal of this clinical trial is to investigate if dosage of Ustekinumab (UST) and Vedolizumab (VDZ) based on Model-Informed Precision Dosing (MIPD) is equally as efficient in keeping adults with Inflammatory Bowel Disease (IBD) in remission as management based on what the treating physician deems best. The main question is:

Is using pharmacokinetic-pharmacodynamic (PK-PD) models to predict the appropriate dose and dosing interval for VDZ and UST at least as effective as current practices in maintaining IBD remission.

As above mentioned the comparison-group is adults with IBD, treated with UST or VDZ, managed as the physician deems best.

Participants will:

Have blood and stool tests done, as well as answer a questionnaire 4th weekly Have their dosage frequency decided on either by the PK-model or as the physician deems best visit the clinic once every 24 weeks for checkups. Have an endoscopy done at completion of the study (if the disease is primarily located in the small intestine, MRI or capsule endoscopy will be used instead)

Patients will 4th weekly have their drug-concentration in blood measured (and if low, also antidrug antibodies). Hemoglobin, Albumin, CRP and white blood cell count will also be done, as well as a stool-sample for calprotectin measurement.

For the control group however, the clinicians and patients will be blinded for the results of the blood- and stool samples, and the samples for drug concentration will not get analyzed before end of study.

4th weekly they will also answer PRO2 questionnaire, as well as the VAS-F. 8th weekly they will in addition to above mentioned also answer the EQ-5D-5L questionnaire.

At inclusion, after 24 weeks, and at 48 weeks the patients will in addition to the above mentioned also answer the short IBDQ and WPAI questionnaires, as well as be seen in the outpatient clinic to obtain HBI or SCCAI score as applicable, as well as to document any changes in concomitant medication or diseases, and to register if any serious adverse events have presented.

At completion of the trial, patients will have a endoscopy done.

In one subprotocol, the investigators will include 20 patients from Odense to have an endoscopy at both inclusion and completion. Here they will also have biopsies taken from all segments of the colon, which will be stores in a biobank for future research.

In another subprotocol 20 patients will be included to have a intestinal ultrasound done at inclusion and completion.

In a biobank for future research the investigators will also store serum-samples, buffy coat, and stool samples collected at inclusion, after 24 and 48 weeks from all included patients.

Inclusion Criteria:

• Ulcerative colitis or Crohn's disease. Diagnosed, according to universally acknowledged criteria, a minimum of 3 months prior to inclusion
• Age ≥ 18
• Stable treatment with VDZ or UST for at least 3 months prior to inclusion
• Stable disease activity, mild activity is accepted, defined by fecal calprotectin ≤ 200, and a weighted PRO2 < 14 for CD or a PRO2 ≤3 for UC
• No change in medical therapy within 3 months prior to inclusion, as concomitant therapy with other immune suppressants is allowed (Azathioprine, 6-mercaptopurine, Methotrexate, 5-aminosalicylic acid)
• The patient must be able to understand patient information material
• The patient must be able to give informed written consent

Exclusion Criteria:

• Having a diagnose of indeterminate colitis
• Having a stoma or pouch
• Fistulizing disease being the primary reason for treatment with VDZ or UST
• Expected eminent change of therapy
• Expected need for surgical intervention within the coming 3 months
• Contraindication against continuing treatment with VDZ or UST, including prior acute or delayed infusion reaction to VDZ or UST
• Any active infection requiring parenteral treatment, known infection with tuberculosis, human immunodeficiency virus (HIV) or hepatitis virus.
• Any condition which the responsible physician finds incompatible with participation in the study
• Patients unable to participate in the collection of symptoms scores
• Patients who are pregnant or nursing at time of inclusion

De-identified individual participant data will possibly be shared, including clinical characteristics, laboratory test results, and patient-reported outcomes relevant to the study endpoints.

Data will be available after publication of the main trial results and completion of any relevant follow-up studies. This is expected to occur approximately one year after the study's primary completion date.

Data will be available for a minimum of 5 years following the publication of the trial results.

will become available approximately March 2028 and will be available for at least 5 years

Data will be shared with qualified researchers conducting legitimate scientific inquiries, particularly those working within the field of inflammatory bowel disease (IBD) and therapeutic drug monitoring (TDM)