The study is conducted to assess the effect of a vitamin D3 supplementation on the activities of two intestinal transporters and on the pharmacokinetics of the transporter substrates folic acid and fexofenadine.

• Trial with medicinal product

Inclusion criteria:

1. male or female
2. age 18 to 65 years at study entry
3. signed informed consent form
4. Body Mass Index > 18 - < 30 kg/m2
5. Participants must not have any other diseases (as assessed by the screening examination)
6. Participants must not take any concomitant medications except oral contraceptives in females
7. Normal blood count

Exclusion criteria:

1. Subjects with confirmed or suspected hypersensitivity towards the study medications
2. Contemporaneous participation in any other study
3. Females only: pregnancy
4. Females only: breast-feeding
5. Clinically significant abnormal laboratory findings
6. Known or suspected present or past malignancies of any kind
7. Known or suspected active infections, serious infections in the preceding 3 months
8. Positive hepatitis B, hepatitis C and / or HIV 1/2 serology
9. Subjects known or suspected not to comply with the study regulations
10. Subjects employed at the participating departments of the University Hospital Zürich.
11. Known or suspected present or past diseases which may interfere with the study
12. smokers
13. alcohol or drug abuse