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Q: What are the different routes of drug administration, and how do they affect drug bioavailability?
A medication administration route is often classified by the location at which the drug is applied, such as oral or intravenous. The choice of routes in which the medications are applied depends not only on convenience but also on the drug's properties and pharmacokinetics*.
These are the various common routes of drug administration
Enteral (gastro-intestinal) Route of Medication
Parenteral Route ( medications placed into the tissues and the circulatory system by injection) of Medication
Other Routes of Medication
1. Oral
This is convenient and indicated for patients who can ingest and tolerate oral medication. Some medications with short half-lives are administered orally as timed-release or sustained-release forms that get absorbed over several hours.
Advantages: Ease of administration and widespread acceptance by the patients.
Disadvantages: Variable absorption rates, degradation of some drugs before reaching the site of absorption into the bloodstream, the inability of many compounds to effectively traverse the intestinal epithelial membrane cells to reach the bloodstream, the insolubility of many drugs at low pH levels is prevalent in the digestive tract, the inactivation of the drug in the liver on its way to the systemic circulation, irritation of the mucous lining of the gastrointestinal tract. This can be prevented to some extent by coating.
2. Sublingual and Buccal Routes
These are indicated for medications with high first-pass metabolism that need to avoid clearance by the liver. For instance, nitroglycerin is cleared more than 90% during a single pass through the liver; therefore, it is given in a sublingual form.
Advantages: Rapid absorption is due to the abundant mucosal network of systemic veins and lymphatics, thereby leading to a rapid onset of action, if there is any untoward event, the tablet can be removed, avoids first-pass hepatic metabolism, a tablet can be kept for a long time in the buccal cavity, which helps develop formulations with a sustained-release effect, this route is useful in patients having swallowing difficulties, low risk of infection, and convenience
Disadvantages: The tablet must be kept in the buccal cavity and neither chewed nor swallowed, excessive salivation may cause quick dissolution and absorption of the tablet, patients may find it difficult to accept an unpalatable tablet. Hence some drugs are applied as a patch or a spray.
3. Rectal Route
This route is useful for patients with gastrointestinal motility problems such as dysphagia or ileus that can interfere with delivering the drug to the intestinal tract. The rectal route is also often utilized in patients near the end of life undergoing hospice care.
Advantages: A relatively large amount of the drug can be administered, those drugs destroyed by the acidic medium in the stomach and those metabolized by pancreatic enzymes can be administered effectively, safe and convenient for infants and the elderly, it can be used in emergency situations, such as in infants having seizures when the intravenous route is unavailable, the rate of absorption is uninfluenced by the ingestion of food or the rate of gastric emptying, bypasses hepatic metabolism, less degradation of drugs compared to that in the upper gastrointestinal tract.
Disadvantages: Some hydrophilic drugs like antibiotics and peptide drugs are unsuitable for rectal administration as they are not readily absorbed, some drugs can cause rectal irritation and proctitis, leading to ulceration and bleeding.
Parenteral Routes of Medication
1. Intravenous Route
This directly administers the medications to the systemic circulation. It is indicated when a rapid drug effect is desired, a precise serum drug level is needed, or when drugs are unstable or poorly absorbed in the gastrointestinal tract. It is also the route utilized in patients with altered mental status or severe nausea or vomiting, unable to tolerate oral medications.
Advantages: Rapid onset of action, predictable way of action and almost complete bioavailability, the problems of oral drug administration can be eliminated by avoiding the gastrointestinal tract, the best way of administration in very ill and comatose patients who cannot ingest anything orally.
Disadvantages: Causes pain, chance of infection, the delivery of protein products that require sustained levels can be difficult.
2. Intramuscular Route
This can be utilized when oral drug absorption occurs in an erratic or incomplete pattern, the drug has high first-pass metabolism, or the patient is not compliant. A depot preparation of the drug can be given intramuscularly, and the medication dissolves slowly into the circulation to provide a sustained dose over a more extended time. An example includes haloperidol decanoate. Vaccines are also administered via the intramuscular route.
Disadvantages: Injection site pain, the amount of drug administered has to be adjusted according to the mass of the muscle available, peptides get degraded locally, complications - hematoma, abscess, peripheral nerve injury, puncture of a blood vessel leading to inadvertent intravascular administration.
3. Subcutaneous Route
This is used when the drug's molecular size is too large to be effectively absorbed in the intestinal tract or when better bioavailability or a faster absorption rate is needed than the oral route. It is easy to administer and requires minimal skills, so patients can often self-administer the medication. Common medications administered subcutaneously include insulin, heparin, and monoclonal antibodies. The rate of absorption of drugs through this route can be enhanced by infiltration with the enzyme hyaluronidase.
The major factors that affect the rate of absorption by this route include the size of the molecules (large molecules having slow penetration), viscosity, and the anatomical characteristics of the site of injection (vascularity and amount of fatty tissue).
Disadvantages: The rate of absorption is difficult to control, local complications - irritation and pain, injection sites must be changed frequently to prevent the buildup of unabsorbed medication, which could lead to tissue injury.
Other Routes of Medication
1. Intranasal Route
This can be utilized in administering nasal decongestants for cold or allergy treatment. Other uses include desmopressin for the treatment of diabetes insipidus or intranasal calcitonin for the treatment of osteoporosis.
Factors that affect the rate of absorption of drugs via the nasal route are: The rate of nasal secretion - The rate of secretion is inversely proportional to the bioavailability of the drug, ciliary movement - The speed of ciliary movement is inversely proportional to the bioavailability of the drug, vascularity of the nose - The volume of blood flow is directly proportional to the rate of drug absorption, metabolism of drugs in the nasal cavity - The enzymes present in the nasal tissues alter the absorption of some compounds, especially peptides that are disintegrated by aminopeptidases, diseases affecting nasal mucous membrane. Common colds can affect nasal drug absorption.
Enhancement of nasal drug delivery:
Rapid mucociliary clearance can lead to poor bioavailability of the drug. This can be overcome by in situ gelling drug delivery. Chitosan is a natural bioadhesive polysaccharide obtained from crustacean shells that can be used as an absorption enhancer. Chitosan binds to the nasal mucosal membrane and facilitates drug absorption through paracellular transport and other mechanisms.
Advantages: Increased permeability of the nasal mucosa compared to the gastrointestinal mucosa, highly vascularized subepithelial tissue, quick absorption, usually within thirty minutes, avoids the first-pass effect, avoids the effects of gastric stasis and vomiting, ease of administration, higher bioavailability of the drugs than in the case of the enteral route or inhalational route.
Disadvantages: Nasal cavity diseases and conditions may result in impaired absorption, the dose is limited due to the small area available for absorption, the time available for absorption is limited, this route does not apply to all drugs.
2. Inhalational Route
The alveolar epithelium is approximately 100 square meters in adults, and the alveolar volume is 4000 to 6000 mL compared to the airway volume of 400 ml. This provides a greater surface area for absorption of the inhaled substances. An inhaled medication route can be utilized to administer albuterol or corticosteroids such as fluticasone and to deliver inhaled anesthetics to patients.
Barriers to the absorption of substances in the alveoli are surfactants, surface lining fluid including the mucus, the epithelial membrane, extravascular space, and vascular endothelium. The absorption of macromolecules by the lungs is considered to be by transcytosis (not significant for macromolecules >40 kDa), paracellular absorption, and drug transporter proteins.
Advantages: Large surface area, proximity to blood flow, avoids first-pass hepatic metabolism, only smaller doses are enough to achieve equivalent therapeutic effects as the oral route
Disadvantages: The aerodynamic filter of the lungs has to be overcome,, clearance of particles by the mucus lining the airways, only 10–40% of the drug from a conventional inhalation device is actually deposited in the lungs.
3. Vaginal Route
This is not commonly used but can deliver low, continuous dosing of medications which can help achieve stable drug levels. A variety of formulations can be given vaginally, including tablets, creams, gels, ointments, and pessaries. Common medications given via the vaginal route include vaginal estrogen therapy for urogenital atrophy, contraceptive rings, antibiotics, or antifungals.
4. Transdermal Route
Iontophoresis and microneedles are playing an increasing role in transdermal drug delivery. A technique has been described using hydrogel-forming microneedle arrays in combination with electro-phoresis for controlled transdermal delivery of biomacromolecules in a simple, one-step approach.
Drug Delivery to the Cardiovascular System
Methods for local administration of drugs to the cardiovascular system are:
Drug Delivery to the Central Nervous System (CNS)
The blood-brain barrier is a major obstruction to the delivery of drugs into the central nervous system.
Some methods to reduce the obstruction by this barrier include:
Each medication administration route has unique contraindications, and the healthcare team members need to recognize them. An oral medication route is contraindicated for patients who cannot tolerate oral drugs, such as those who have altered mental status or have nausea or vomiting that hinder them from safely ingesting the drug orally. A rectal route is contraindicated in patients with active rectal bleeding or diarrhea or after a recent rectal or bowel surgery. An intramuscular route is contraindicated in an active infection or inflammation at the site of drug administration, myopathies, muscular atrophy, thrombocytopenia, or coagulopathy.
A subcutaneous route of medication is contraindicated in an actively infectious or inflamed site. Doses that require to be injected more than 1.5 mL at once should be avoided. Subcutaneous injection volumes larger than 2 mL are associated with adverse effects, including pain and leakage at the injection site. An intranasal medication is contraindicated in patients with nasal trauma, anatomic obstruction, the presence of a foreign body, or copious mucous or bleeding. Similarly, an inhaled drug is contraindicated in patients with airflow obstruction.
Source:
Each patient is thoroughly studied and the route of drug administration is decided based on patient’s condition, the drug potency and bioavailability values by the medical team.
Also other medicines taken by the patient also interact and impact the outcome (1).
When a person 's prescribed several different medications at once in order to treat one or multiple health conditions—the phenomenon is known as polypharmacy.
When people take several medicines ... they can (1) ...
1. Interact with one another
2. produce over dose of a drug because too much of one drug remains in your system because of the interactions
2. reduce or increase the potency of one another if they act in opposite or similar ways
3. nullify one another's potency if one drug can’t be absorbed or metabolized properly
4. produce other toxic and harmful products after reacting with one another inside human bodies
5. interfere with the patient's normal metabolic processes
6. cause damage to body organs
7. cause several other severe health conditions as a result of the above processes
-and this one is a positive one-
8. successfully suppress an existing health condition and help us find a new cure for diseases that don't have medicines yet!
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