The fall of viscous slugs in vertical capillary tubes is described. Deviations toward Poiseuille law are analyzed by taking into account the dissipation in menisci, together with the existence of a film behind the slug. Slugs are found to fall slower in dry tubes than in prewetted ones, which is quantitatively discussed in term of viscous friction. A criterion for the minimal length of the slug obeying the Poiseuille solution is finally derived.