Drugs are generally employed as replacement therapy, as agonists, or
as antagonists. Drugs are used as replacement for conditions in which
there is deficiency or a natural substance-endogenous, e.g, hormones, or
exogenous, e.g, nutrients.
Agonist drugs produce a pharmacological effect, e.g, hypotensives,
analgesics. Antagonist drugs prevent the action of natural substances
like anticholinergics or counteract the action of other drugs, e.g,
nalorphine. Drugs produce their effect by interacting with specific
receptor sites on the cell membrane. At the receptor sites the action
may be positive (e.g, agonists) or opposite to those of agonists (e.g,
antagonists). Initially there is physical interaction between the drug
and the receptor and this drug receptor complex produces the therapeutic
response.
Routes of drug administration: This may be oral,
sublingual, topical or parenteral. Parenteral routes are subcutaneous,
intra-muscular, intravenous, intra-arterial, intrathecal,
intra-ventricular (cerebral ventricles), intra articular, intracavitary
(into viscera like pleura or peritoneum or abscess cavities), and
intra-amniotic.
Absorption of Drugs
The amount of drug that reaches the systemic circulation intact and
is available to the target tissues for effective function is called the
"bio-availability" of a drug. The rate of absorption, and the metabolic
processes which tend to eliminate or inactivate the drug determine the
bio-availability of the ingested drugs.
Absorption from the
gastro-intestinal tract depends upon lipid solubility, degree of
ionization, and molecular weight of the drug. Low molecular weight
substances and alcohol are absorbed passively, while other drugs are
absorbed by active processes requiring energy, e.g, apha methyl dopa.
Lipid-soluble drugs are absorbed easily. Since drugs have to be in
solution to facilitate absorption, formulation which disintegrate
rapidly, are more easily absorbed. Due to the variability of physical
properties, bio-availability of the same drug may vary when administered
in different formulations and therefore it is important to avoid
frequent changes in formulations from time to time.
Other factors
influencing gastrointestinal absorption are the availability of surface
area for absorption, motility of the gut, pH within the gut, local blood
flow, and presence of other substance in the gut lumen.
Drug
interactions in the gut may alter the rate and magnitude of absorption.
For example, antacids reduce the absorption of Iron by changing the pH
and anticholinergics reduce the absorption of other drugs due to delay
in gastric emptying. Drug absorption is delayed by the presence of food.
For several drugs, administration with water on an empty stomach
ensures maximum absorption is erratic. In congestive heart failure
venous stasis occurring in the intestines impairs absorption. Drug
absorption is unpredictable in the elderly.
sublingual administration
Drugs which are better
absorbed from buccal mucosa are preferably given sublingually. Some
drugs like notroglycerine and isoprenaline are destroyed by the gastric
acid and hence sublingual administration is ideal. Usually, the clinical
effect is evident within minutes. Another advantage of sublingual
administration is that, drug toxicity can be avoided by removing the
drug from the mouth or swallowing it as soon as the desired effect is
achieved.
Topical routes
include the skin and the mucous membranes
of the nose, rectum and lungs. Preparations for topical application
include inunctions (for example, nitroglycerine), suppositories into the
rectum (for example, indomethacin), nasal medications (for example,
pitressin) and aerosols which are inhaled.
Rate of absorption of
topically applied drugs depends on the concentration, lipid solubility
and local blood flow. Absorption is lower from Keratinized epithelium.
The size of the particle is important when drugs are applied as
aerosols. Only particles, below 2 nm in size reach the alveoli.
Absorption from the respiratory epithelium is rapid and the effect is
immediate. The portion deposited in the oropharynx is later swallowed
and absorbed to produce mild and delayed effect. Considerable attention
to details is necessary for ensuring proper administration of the
aerosol.
Parenteral administration
is resorted to when more rapid
action is desired and when patient's co-operation cannot be relied upon.
Local vascularity influences absorption from sites of intra-muscular or
subcutaneous injections.
Aqueous formulations are absorbed rapidly
whereas oily preparations are absorbed only slowly. Local warmth and
massage favour absorption.
Intramuscular injections are usually given
into the deltoid, rectus abdominis or gluteal muscles.
Certain drugs
like diazepam and phenytoin may be erratically absorbed, so that oral
administration may be more reliable than intramuscular doses.
The
subcutaneous route is to be preferred when the volume of drug is small
and drug is non-irritant. The rate of absorption is slightly lower that
the intramuscular route. When the peripheral circulation is
insufficient, subcutaneous and intra-muscular routes are unreliable.
Intravenous route should be resorted to when the drug action has to be
immediate, or large volumes or irritant drugs have to be given. Since
the onset of action is immediate, the pharmacological effect can be
adjusted by controlling the rare of intravenous infusion.
Certain drugs
like digoxin take several minutes to exert their full effect even after
intravenous administration.
Highly irritants drugs like nitrogen
mustards can be given into a rapidly flowing rip since the drug is
diluted and the venous walls are relatively resistant. Local damage to
vein can be avoided by releasing the drug into more central portions of
the circulation, e.g, inferior vena cava. Intra-arterial infusions are
indicated when a large dose of drug has to be given in a high
concentration at a particular site.
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