The present invention relates to a process for producing a paper or paperboard product which process comprises the steps of providing a first headbox with a furnish comprising fibers, which furnish forms at least one layer of the paper or paperboard product, providing the headbox with a non-fibrous high consistency composition, which composition forms at least one additional layer of the paper or paperboard product and conducting the layers onto a wire forming a multilayer web which is further dewatered and dried forming the paper or paperboard product. The invention further relates to a paper or paperboard produced according to the process and a headbox used in the process.