The capillaries are typically lined with endothelium whose cells overlap, though to a lesser degree than epithelial cells. Also, the junctions between cells are discontinuous.
Capillary walls are quite permeable.
Lipid soluble drugs pass through very rapidly. Water soluble compounds penetrate more slowly at a rate more dependent on their size. Low molecular weight drugs pass through by simple diffusion. For drugs which can be ionized the drug's pKa and the pH of the blood will have a large effect on the transfer rate across the capillary membrane. There are two deviations to the typical capillary structure which result in variation from normal drug tissue permeability.