Sublingual and buccal administration
Administration of 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 extremely poorly via this route.
Advantages of the method:
- High bioavailability. The substance enters the systemic circulation directly, bypassing the liver. This prevents presystemic metabolism and biliary excretion of the drug (no first-pass effect).
- Rate of action. The intensive blood supply of the oral mucosa ensures a rapid onset of effect, which is critically important in emergency situations.
- Stability. The drug is not subjected to the destructive action of gastric hydrochloric acid.
- Convenience. High compliance and ease of use for the patient.
The main limitation of this route is the small surface area for absorption. The method is suitable exclusively for highly potent substances used in very doses. Classic examples include relieving an angina attack with Nitroglycerinum (0.5 mg tablets), lowering blood pressure during a hypertensive crisis (Clonidinum, Captoprilum), or treating acute diarrhea (Loperamidum).
Oral administration (Per os)
Taking medications by mouth is the most common method due to its convenience, lack of sterility requirements, and the ability to undergo treatment without the participation of qualified medical staff.
Where does absorption take place? 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 too brief contact with the mucosa. The drug reaches the rectum (rectum) already in a low concentration.
The absolute leader in the absorption of any drugs is the small intestine. Its dominance over the stomach is due to three anatomical factors:
- A huge surface area of the mucosal absorption surface (around 200 m²).
- A smaller thickness of the histological epithelial barrier.
- Powerful vascularization (blood supply), which constantly maintains the required concentration gradient.
Transport mechanisms:
- Passive diffusion: The main pathway. To pass through epithelial cell membranes, the 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 the participation of specific transport proteins.
Challenges and limitations of the oral route
Despite its convenience, the per os route has a number of significant drawbacks. Drug efficacy can vary greatly depending on the condition of the mucosa, intestinal motility, pH levels, and food composition (chyme). In addition, part of the dose is lost due to the first-pass effect, when the substance enters the liver via the portal vein system and is partially inactivated.
Interaction with the digestive system can be bidirectional. Drugs can disrupt the process of digestion and nutrient assimilation. For example, osmotic laxatives hinder the absorption of nutrients from the intestinal lumen, while antacids neutralize hydrochloric acid ($HCl$), which disrupts 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 delivering the drug to the GI tract are used:
- Tube feeding: Performed using a thin gastric tube passed 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, allowing several limitations of the oral route to be avoided.
Briefly: answers to topical questions
What are the pharmacokinetic advantages of the rectal route of administration over the oral route?
The provided sources do not contain a direct description of the pharmacokinetic advantages of the rectal route of administration over the oral route. They indicate that rectal administration is considered an alternative to oral administration in specific clinical situations, and also describe blood drainage from the lower part of the rectum through hemorrhoidal veins into the systemic circulation via portosystemic shunting.
What dosage forms are used for rectal administration of drugs?
Suppositories, rectal foam, and enemas are used for rectal administration. Suppositories are solid at room temperature and melt at body temperature; the mass of rectal suppositories is 3.0 g. Rectal foam is used, in particular, with mesalazine and budesonide. Mesalazine is also administered rectally in the form of enemas.
Class and exam questions
What is meant by the "first-pass effect"?
This is the process of partial inactivation of a drug substance in the liver. After absorption in the GI tract, the drug enters the portal vein system and passes through the liver before reaching the systemic circulation.
Why can only microdoses of drugs be administered sublingually?
The oral mucosa has a very small absorption surface area. Therefore, this route is suitable exclusively for highly potent substances that act in minimal concentrations.
What molecular properties determine a drug's ability to undergo passive diffusion in the intestine?
The determining property is lipophilicity and the absence of polarity. Hydrophilic polar molecules pass extremely poorly through the small intercellular spaces of the GI epithelium.
What else to study on this topic
- Features and clinical benefits of the rectal route of administration (Per rectum)
- Active transport of hydrophilic substances: the role of transport proteins
- The effect of chyme and gastric pH on drug bioavailability
- Technique and indications for drug administration via a thin gastric tube
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