Sublingual and Buccal Administration
Administering drugs under the tongue (sublingualis) and against the cheek (transbuccalis) relies on passive diffusion through the oral mucosa. For successful absorption, molecules must be lipophilic and nonpolar; polar hydrophilic compounds penetrate this barrier extremely poorly.
Advantages of the method:
- High bioavailability. The substance enters the systemic circulation directly, bypassing the liver. This prevents presystemic metabolism and biliary excretion (no first-pass effect).
- Rapid onset of action. The dense vascular supply of the oral mucosa ensures a quick onset of effect, which is critical in emergency situations.
- Stability. The drug avoids the destructive action of gastric hydrochloric acid.
- Convenience. High patient compliance and ease of use.
The main limitation of this route is the small surface area available for absorption. The method is strictly suited for highly potent substances administered in very low doses. Classic examples include terminating an anginal attack with Nitroglycerin (0.5 mg tablets), lowering blood pressure during a hypertensive crisis (Clonidine, Captopril), or treating acute diarrhea (Loperamide).
Oral Administration (Per os)
Taking medications by mouth is the most common method due to its convenience, lack of sterility requirements, and the ability for patients to self-administer without medical personnel.
Where does absorption occur? The primary sites of absorption are the stomach (gaster) and the small intestine (intestinum tenue). Absorption in the oral cavity and esophagus (oesophagus) during normal swallowing is negligible due to brief mucosal contact. By the time a drug reaches the rectum (rectum), its concentration is already low.
The small intestine is the absolute leader in drug absorption. Its dominance over the stomach is due to three anatomical factors:
- A massive mucosal surface area (around 200 m²).
- A thinner histological epithelial barrier.
- Robust vascularization (blood supply), which constantly maintains the required concentration gradient.
Transport mechanisms:
- Passive diffusion: The primary pathway. To cross epithelial cell membranes, a drug requires sufficient relative lipophilicity. Hydrophilic polar substances are absorbed with difficulty due to the small intercellular spaces in the GI epithelium.
- Active transport: Operates for certain hydrophilic substances (e.g., Levodopa), requiring specific transport proteins.
Challenges and Limitations of the Oral Route
Despite its convenience, the per os route has several significant drawbacks. Drug efficacy can vary widely depending on the condition of the mucosa, intestinal motility, pH levels, and the composition of food (chyme). Additionally, a fraction of the dose is lost due to the first-pass effect, where the substance enters the portal venous system and is partially inactivated in the liver.
Interactions with the digestive system can be bidirectional. Medications can disrupt digestion and nutrient absorption. For example, osmotic laxatives impair nutrient absorption from the intestinal lumen, while antacids neutralize hydrochloric acid ($HCl$), disrupting normal protein denaturation and digestion.
Contraindications to oral administration:
- Unconscious state of the patient.
- Impaired swallowing (dysphagia).
- Patient refusal of treatment.
- Early childhood (due to difficulties swallowing solid forms and aspiration risk).
Alternative Enteral Routes
If oral administration is not possible, alternative methods of drug delivery to the GI tract are used:
- Tube feeding/administration: Performed using a thin gastric tube inserted through the mouth or nasal passages. The target area is the stomach (gaster) and/or duodenum (duodenum).
- Rectal administration (Per rectum): Used as an effective alternative in specific clinical situations, avoiding several limitations of the oral route.