Gaseous plasma is a suitable medium for treatment of polymer products. While the uniform treatment of flat objects like foils or textiles has been industrialized decades ago the treatment of inner walls of narrow long tubes like catheters remains a challenge. Atmospheric pressure discharges perform well in long tubes of diameter up to few mm as long as they are created in helium, but cannot be sustained in reactive gases suitable for surface functionalization and nanostructuring. Low pressure plasma can be sustained in narrow dielectric tubes by microwave discharges but the required power density is too large due to insufficient frequency of microwave generators since the plasma electrons fail to acquire enough energy between two subsequent collisions with gaseous molecules to be energetic enough for causing ionization collisions. An ideal solution would be application of terahertz technology. Current state-of-the-art on plasma treatment of narrow dielectric tubes will be presented and the needs for developing a terahertz discharge stressed.