Routes of drug administration
In clinical practice the physician commonly encounters two important
questions:
1-What is the best drug to be used?
2-How it is going to be given?
The answer to the second question could be as important as the first one,
because the rout of drug administration could have a profound effect on the
overall outcome of treatment.
Generally there are two main routes of drug administration:
1-Entral route (i.e. through the gastrointestinal tract)
2-Parentral (i.e. outside the gastrointestinal tract)
But in our daily practice the term parental is reserved for injections; that is
why we can add a third category called:
3- Other routes of administration. (Topical, Inhalation…..etc.)
1-Entral route of administration:
A-Oral rout:
Advantages:
1- Convenient - portable, no pain, easy to take.
2- Cheap - no need to sterilize (but must be hygienic of course), compact,
multi-dose bottles, automated machines produce tablets in large
quantities.
3- Variety - fast release tablets, capsules, enteric coated, layered tablets,
slow release, suspensions, mixtures…etc.
Disadvantages:
1- Sometimes inefficient - high dose or low solubility drugs may suffer
poor availability; only part of the dose may be absorbed.
2- First-pass effect - drugs absorbed orally are transported to the general
circulation via the liver. Thus drugs which are extensively metabolized will be
metabolized in the liver during absorption.
e.g. the propranolol oral dose is somewhat higher than the IV, the same is true
for morphine. Both these drugs and many others are extensively metabolized in
the liver.
3- Food and G-I motility can effect drug absorption. Often patient instructions
include a direction to take with food or take on an empty stomach. Absorption is
slower with food for tetracyclines and penicillins, etc. However, for propranolol
bioavailability is higher after food, and for griseofulvin absorption is higher
after a fatty meal.
4-Local effect - Antibiotics may kill normal gut flora and allow overgrowth
of fungal varieties.
5-Unconscious patient - Patient must be able to swallow solid dosage
forms. Liquids may be given by tube.
B-Buccal/Sublingual:
Some drugs are taken as smaller tablets which are held in the mouth or under
the tongue. These are buccal or sublingual dosage forms. Buccal tablets are
often harder tablets [4 hour disintegration time], designed to dissolve slowly.
Nitroglycerin, as a softer sublingual tablet [2 min disintegration time], may be
used for the rapid relief of angina.. Nicotine containing chewing gum may be
used for cigarette smoking replacement.
Advantages:
1- First pass - The liver is by-passed thus there is no loss of drug by first
pass effect for buccal administration. Bioavailability is higher.
2- Rapid absorption - Because of the good blood supply to the area
absorption is usually quite rapid.
3- Drug stability - pH in mouth relatively neutral (cf. stomach - acidic).
Thus a drug may be more stable.
Disadvantages:
Holding the dose in the mouth is inconvenient. If any is swallowed that portion
must be treated as an oral dose and subject to first pass metabolism. Small doses
only can be accommodated easily.
C-Rectal: (suppository or enema.)
Some drugs are commonly given by this route including indomethacin,
theophylline metoclporomide, diclofenac.
Advantages:
1- By-pass liver - Some of the veins draining the rectum lead directly to the
general circulation, thus by-passing the liver.
2- This route may be most useful for patients unable to take drugs orally or
with younger children.
Disadvantages:
1- Erratic absorption - Absorption is often incomplete and erratic.
However for some drugs it is quite useful.
2- Not well accepted
2-Parentral route of administration (IV. IM. SC.)
Subcutaneous (SC.)
This involves administration of the drug dose just under the skin. Advantages:
1- Can be given by patient, e.g. in the case of insulin
2-
Absorption slow but usually complete. Improved by massage or heat.
Vasoconstrictor may be added to reduce the absorption of a local
anesthetic agent, thereby prolonging its effect at the site of interest
.
Disadvantages:
1- Can be painful
2- Irritant drugs can cause local tissue damage
3- Maximum of 2 ml injection thus often small doses limit use.
Intramuscular(IM.)
Advantages:
1- Larger volume, than sc, can be given by IM
2- A depot or sustained release effect is possible with IM injections, e.g.
procaine penicillin
Disadvantages:
1- Trained personnel required for injections. The site of injection will
influence the absorption, generally the deltoid muscle is the best site
2- Absorption is sometimes erratic, especially for poorly soluble drugs, e.g.
diazepam, The solvent maybe absorbed faster than the drug causing
precipitation of the drug at the site of injection.
Intravenous (IV.)
Drugs may be given into a peripheral vein over 2 to5 minutes or longer by
infusion. Intravenous injections are often used in emergencies e.g. epileptic
seizures, acute asthma, or cardiac arrhythmias.
Diagram
Showing IV, IM, and
SC Injection
Advantages:
1- Rapid - A quick response is possible
2- Total dose - The whole dose is delivered to the blood stream. Large doses
can be given by extending the time of infusion.
3- Veins relatively insensitive to irritation by irritant drugs at higher
concentration in dosage forms.
Disadvantages:
1- Suitable vein - It may be difficult to find a suitable vein.
2- Maybe toxic - Because of the rapid response, toxicity can be a problem
with rapid drug administrations.
3- Requires trained personnel - Trained personnel are required to give
intravenous injections.
4- Expensive - Sterility, pyrogen testing and larger volume of solvent means
greater cost for preparation, transport and storage.
3-Other routes of administration
Inhalation
Absorption of gases is relatively efficient, however solids and liquids are
excluded if larger than 20 micron and even then only 10 % of the dose may be
absorbed. Particles larger than 20 micron impact in the mouth and throat. This
route of administration can be used for:
1- Local effect - bronchodilators
2- Systemic effect - general anesthesia
Topical
1- Local effect - eye drops, creams, ointments, etc.
2- Systemic effect - e.g., nitroglycerin transdermal patch.
Intra articular injection:
This means injecting the drug into the synovial fluid to get a direct local action
of the drug into the joint space, e.g. local steroid hormone injections.
Itrathecal injections:
This means injecting the drug into the cerebrospinal fluid (CSF), through a
special CSF needle by making a puncture through L3-L4, or L4-L5,
intervertebral space.
This method is commonly used in the setting of chemotherapy regimen in CNS
prophylaxis or treatment.
Epidural injection:
This technique is somewhat similar to the intrathecal injection, except that the
injection is given in the epidural space.
It is commonly used to give local analgesia during labor.
Itraplueral &intraperetonial injection:
Means injecting the drug directly into the pleural space to induce pleurodesis
(obliteration of the pleural space), or to give chemotherapy directly to the
pleural or peritoneal space.