Introduction: multivitamins are normally used to treat vitamin deficiencies. They may cause constipation and diarrhea. We are concerned to see the prevalence for using multivitamins supplementation & awareness of their side effects among saudi people. Methods: we conducted a cross sectional study and data collected by distributing an electronic questionnaire from 6/3 to 15/8/2017 and approved by the King Faisal University in Alhass, SA. Both gender above 18 years were included and excluded people below them. Statistical analyses were performed using SPSS. Results: this study was applied on 471persons from different areas of Kingdom of Saudi Arabia. Most of them were female 67.5 %.the age of our studied group was above 18 years. Most of them was healthy, pre-university, from western area and used multivitamins for weight gain. 15.5% had harmful effect and 69.6% had vitamins or minerals deficiency (51.7% iron deficiency). 43.1% used them with no prescription and 85% used as pill. 80.8% considers them are important to their health and 35.8% believe there is no difference between using vitamins with or without medical advice. 24.9% believe they have no side effects and16.3% believe that they are sufficient for the variety and quantity of food.70.7% believe that the amount of vitamins in the food is not sufficient for the daily body and 42.5% read about vitamins from scientific books.  Conclusion: the majority of the participants were using vitamins according to medical advice and there is a less percentage of people that experienced a side effect from using that supplementations. Objectives: to determine the prevalence, reasons, source of information of multivitamins supplement uses, and their side effects.