Gaseous substances like general anesthetics are absorbed and excreted through the lungs via simple diffusion. This process depends on factors such as pulmonary blood flow, respiration rate, and the substance's solubility. Gaseous anesthetics like nitrous oxide with low blood solubility are excreted rapidly, while compounds like alcohol, with high blood and tissue solubility, are excreted slowly.

Drugs can also be excreted in breast milk, which is crucial for breastfeeding infants. The process is passive and depends on factors like pH, molecular weight, lipid solubility, and degree of ionization. Due to milk's acidic nature compared to plasma, weakly basic drugs concentrate more in milk. However, potent drugs like barbiturates, morphine, and ergotamine may induce toxicity in infants. This means that nursing mothers should avoid medication if possible.

Lastly, drug excretion in saliva is also a passive diffusion process. While the salivary concentration of many drugs can reach approximately 0.1%, certain drug classes, such as cephalosporins, have been shown to achieve significantly higher concentrations in saliva. The blood concentration of several drugs can also be determined by detecting their amount in saliva. However, this process can leave a bitter aftertaste or cause dryness in the mouth.