Understanding the Subcutaneous Administration Route
The subcutaneous (SC) route involves injecting medication into the fatty tissue layer located just beneath the skin. This method is favored for its slow and sustained absorption, which is ideal for drugs requiring consistent delivery over time, such as insulin and heparin. However, several factors limit its utility, making it an unsuitable route for a number of medications. A primary constraint is the small volume of fluid that can be injected at one site, typically limited to 1.5–2 mL to prevent pain and tissue damage. Additionally, the rate of absorption is significantly slower compared to intramuscular (IM) or intravenous (IV) injections, due to a less extensive blood supply in the subcutaneous fat.
Drugs Unsuitable for Subcutaneous Injection
Identifying drugs that are inappropriate for SC administration is crucial for patient safety. The following are key examples of drugs that are typically not given via this route due to their properties or desired therapeutic effect.
Drugs That Cause Severe Tissue Irritation or Necrosis
- Methadone: While many opioid analgesics can be given subcutaneously, methadone is a notable exception. It is known to cause significant local irritation, including induration (hardening) and erythema (redness). Though it has been used in some palliative care settings with extreme caution and frequent site rotation, it is not recommended for routine SC administration.
- Prochlorperazine (Stemetil) and Diazepam (Valium): These medications are known to cause severe local tissue reactions and necrosis when injected subcutaneously. As such, they are typically administered via intramuscular or intravenous routes when an oral form is not suitable.
Medications Requiring Rapid Onset of Action
- Concentrated Vasopressors: Emergency medications used for conditions like shock or severe hypotension, such as epinephrine or norepinephrine, require an immediate and predictable systemic effect. The slow, erratic absorption of the SC route is a major drawback in these critical situations. Therefore, these drugs are always administered intravenously to ensure rapid delivery directly into the bloodstream.
Solutions with Unfavorable Properties
- Hypertonic Solutions: Solutions with a high concentration of solutes, such as 5% dextrose in water, are not recommended for SC administration. They can cause discomfort and disrupt the local electrolyte balance.
- Large-Volume Solutions: Drugs requiring administration in volumes greater than 2 mL often cannot be given subcutaneously due to the risk of pain, pressure, and impaired absorption. For these situations, IV administration is the standard of care.
Comparison of Administration Routes
The choice of injection route is a balance of therapeutic need and drug properties. The following table provides a comparison of the key features of subcutaneous, intramuscular, and intravenous injections.
| Feature | Subcutaneous (SC) | Intramuscular (IM) | Intravenous (IV) |
|---|---|---|---|
| Absorption Rate | Slow and sustained | Faster than SC due to better muscle blood supply | Immediate, directly into bloodstream |
| Maximum Volume | Typically 1.5–2 mL | Larger than SC, up to 5 mL in large muscles | No volume limit; can be large |
| Onset of Action | Slower than IM/IV | Rapid | Instantaneous |
| Local Irritation Risk | Low for compatible drugs, high for irritants | Moderate, but muscle tissue is more tolerant | Bypasses tissues, low risk of local irritation for most IV drugs |
| Typical Drugs | Insulin, Heparin, some vaccines | Vaccines, Hormones, some antibiotics | Antibiotics, Chemotherapy, Emergency drugs |
Physiological and Pharmacological Factors Limiting SC Use
Beyond individual drug properties, broader physiological factors determine whether the SC route is appropriate. These considerations highlight why the route is not a universal solution for injectable medications.
Pharmacokinetic and Pharmacodynamic Limitations
The most significant limitation is the variable and often slow absorption rate from the SC tissue. In emergencies, a slow onset of action is unacceptable. For instance, a drug needed to stabilize a patient in shock cannot rely on absorption from the subcutaneous fat; it must be delivered directly into the circulation via the IV route for immediate effect.
Patient-Specific Contraindications
Certain patient conditions can make SC administration unsafe or ineffective. In patients with severe dehydration or hypothermia, peripheral vasoconstriction can dramatically reduce blood flow to the subcutaneous tissue, severely hampering drug absorption. In cases of generalized edema, drug absorption can also be unreliable. Patients with coagulation disorders may be at higher risk of bleeding or hematoma formation at the injection site when given SC injections, particularly of anticoagulant medications.
Limitations of Molecular Structure
For some medications, particularly large, complex biological molecules like monoclonal antibodies, absorption from the SC space can be incomplete and variable. While modern formulations and enhancers like hyaluronidase have enabled SC delivery for some biotherapeutics, many are still best administered via the IV route for consistent and reliable systemic exposure.
Conclusion
To answer the question, "Which of the following drugs is not given via subcutaneous route?" one must consider several critical pharmacological principles. Drugs that are highly irritating (like methadone, prochlorperazine, or diazepam), require a rapid onset of action (like concentrated vasopressors), or are delivered in large volumes are unsuitable for this route. The limitations of the subcutaneous space—namely restricted volume, slower absorption, and potential for tissue damage—mandate the use of alternative routes, such as intramuscular or intravenous injections, to ensure patient safety and therapeutic efficacy.
For more information on drug administration routes and pharmacology, consult authoritative sources like the National Institutes of Health (NIH) at www.ncbi.nlm.nih.gov.
