Gaseous, inhaled and special-route forms

When a drug is swallowed, it is absorbed into the blood of the gut wall and then travels through the portal vein to the liver before reaching the general circulation. The liver is a detoxifying organ, so a fraction of the dose is chemically changed or removed before it can act; this is first-pass metabolism. The inhaled route avoids that first pass, because the drug is absorbed across the respiratory epithelium and enters the bloodstream downstream of the liver. The consequence is a rapid action with a smaller dose and fewer systemic side effects, which is why some drugs are administered only in this way. The physical requirement is that the particles be small enough to travel into the airways: the notes specify droplets of 50 micrometres in diameter or less, released through a metering valve. The vocabulary worth learning is metered-dose inhaler, propellant, and nebulizer. A beginner’s misconception is that a nebulizer must be better than an inhaler because it looks more medical. The notes state the opposite: inhalers are generally preferred because they are cheaper, more portable and carry less risk of side effects, and nebulizers are reserved for serious respiratory disease or severe attacks. A second misconception is that the inhaled route is precise. The notes list inaccurate dosage intake among its disadvantages, so a mistake in breathing technique changes how much drug actually arrives, even when the device works perfectly.

Special routes are chosen because they solve a specific problem, and each brings its own rule. The eye has almost no barrier against introduced organisms, so all eye preparations must be administered aseptically: sterile aqueous isotonic drops, or sterile semi-solid ointments and gels. The nose is lined with cilia that sweep the airway clean, so oily nasal drops are avoided because they may damage those cilia; aqueous drops may act locally as a decongestant or be absorbed through the nasal mucosa for a systemic effect. The rectum is useful for babies and whenever oral dosing is not practical, and it offers both local action through suppositories and ointments and systemic absorption; enemas then divide by purpose, the evacuant type used as a bowel stimulant with volumes up to two litres and warmed to body temperature, and the retention type kept under 100 ml and not warmed, such as the barium enema for imaging or the nutrient enema for systemic supply. The vagina accepts pessaries of three designs plus the doughnut-shaped vaginal ring for controlled release and the intrauterine device, a birth-control device placed in the uterus and approved for five to ten years depending on type, while douches are aqueous solutions that may or may not be medicated. The unifying principle is that a mucous membrane can serve either local treatment or systemic absorption; which one you get depends on the drug, the formulation and the site. The beginner’s misconception is that a suppository is by definition a local treatment, when the notes explicitly state that rectal suppositories are used for local and systemic effects.