Because the cell membrane is lipoid, lipid-soluble drugs diffuse more rapidly than relatively lipid-insoluble drugs.Small molecules tend to penetrate membranes more rapidly than large ones.

• 由于细胞膜为类脂质，脂溶性药物的扩散比相对的非脂溶性药物更快些，小分子的透膜速度也要比大分子快。