ROUTES OF DRUG ADMINISTRATION
Often there is a great choice in selecting the
route by which a drug should be given to
patients. However, the condition of the
patient and knowledge of advantages and
disadvantages of various routes are of prime
importance in making the selection of best
suiting route.
Following are the different ways by which
a therapeutic agent could be given to
patients.
1 Enteral
2 Parenteral
3 Inhalation
4 Topical or Local
1.
i.
ENTERAL
ORAL OR PER OS (P.O.)
Oral route is the most common route of
administration. It is safe, convenient, cheap
and does not require the services of a skilled
personnel. However, it has certain
disadvantages.
a) Some drugs are unpalatable and cause
irritation of the intestinal tract resulting in
nausea, vomiting and diarrhea, in particular
if these are given before meal.
b) Some drugs are destroyed by intestinal
enzymes e.g. insulin is destroyed by
intestinal enzymes.
c) In case of emergency, when quick action of a
drug is desired this route is not suitable.
d) This route is not suitable in the cases of
unconscious patients.
e) There is a necessity for cooperation on the
part of patient.
f) Absorption may be slow, unpredictable and
irregular because of the presence of variable
amounts of food at various stages of digestion
and acidity and alkalinity of the digestive
juices might have a great impact on
absorption of drugs.
g) A very important factor is that blood from
intestinal tract passes via portal vein to the
liver where the drug may be metabolized to a
great extent before being distributed to the
site of action. Thus oral route is not
recommended for drug undergoing extensive
FIRST PASS EFFECT.
FIRST PASS EFFECT
First Pass Effect may be defined as the loss of
drug as it passes through the gastrointestinal
membranes and the liver, for the first time,
during the absorption process after oral
administration. This is also known PreSystemic elimination.
h) Drugs interaction may occur if two drugs are
given cocurrently.
ii
SUBLINGUAL
The tablet is placed under the tongue and
absorption form oral mucosa is rapid and
uniform. This route has special importance for
certain drugs. For example nitroglycerine is
effective when given sublingually but
ineffective when administered orally. The
reason is that the drug has very high lipid
solubility.
Also the drug being very potent needs few
molecules to be absorbed in order to produce
the therapeutic effect. The major advantage
of this route is that venous drainage from
mouth (bucal cavity) is poured into the
superior vena cava and the drug is saved from
first-pass effect. It nitroglycerine is given by
oral route, the hepatic first-pass effect is
sufficient to preclude the appearance of any
intact nitroglycerine in the systemic
circulation.
iii) RECTAL ADMINISTRATION
The drug may be given rectally for systemic
effect when the patient is either
unconscious or vomiting. However,
absorption from rectum is irregular and
incomplete and may cause irritation of
rectal mucosa: Also 50% of the drug
absorbed from rectum passes through liver
before entering the systemic circulation thus
first-pass effect cannot be fully avoided.
The drugs administered reactally are in the
form of suppositories e.g. Ergotamine for the
treatment of migraine. Another form of
preparation for rectal administration is the
ENEMA i.e. a solution or suspension of the
drug in water or some other vehicle.
Suppositories may also be given for local
treatment of rectal conditions e.g. benzocain
is used to relieve pain and itching caused by
haemorrhoids
Advantages of rectal Administration:Drugs could be given by this route in vomiting,
motion sickness, migraine or when the
patients is unable to swallow the medication.
This route is also suitable for drugs that are
irritant to the stomach e.g. aminophylline,
indomethacin. This route is of particular value
in case of children.
Disadvantages of Rectal Administration:a) The patient may be embarrassed.
b) Inflammation of rectum may occur due to
repeated administration.
c) The absorption is irregular specially when
rectum is not empty.
2-
PARENTERAL
(Par-beyond enteral-intestine)
The term parenteral administration implies the
routes through which the drug directly reaches
the body fluids, by passing the preliminary
process of transport through the intestinal wall
or pulmonary alveoli which is an essential
process when drugs are taken orally, inhaled or
administered reactally. Following are the
Parenteral routes
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
Subcutaneous
(S/C)
Intramuscular
(I/M)
Intravenous (I/V)
Intraperitoneal (I/P)
Intradermal
Intra Medullary
Intrathecal
Intraarticular
Intra-cardiac
Intra arterial
The Parenteral administration has certain
advantages over oral route.
i) Drug is neither invaded nor destroyed by
digestive enzymes.
ii) A higher concentration of drug in blood may
be achieved because the hepatic
metabolism of drug due to First-Pass effect
is avoided.
iii) Absorption is complete and predictable.
iv) In emergency this method is particularly
useful. If the patient is unconscious,
uncooperative or vomiting, the Parenteral
therapy becomes necessary.
However, there are certain disadvantages of
the parenteral therapy which are as under:
i) It is expensive because all the parenteral
preparations should be sterilized.
ii) Asepsis must be maintained to avoid
infection.
iii) An intravascular injection may accidentally
occur when it is not actually intended.
iv) Pain may accompany or follow the injection.
v) It requires the services of a professionally
skilled personnel because it is difficult for
the patient to perform the injection himself.
a) Subcutaneous:The drug is dissolved in a small volume of
vehicle and injected beneath the skin from
where the absorption is slow and uniform.
Substances causing irritation to the tissues
should not be injected otherwise they will
cause pain and necrosis (deadening of
tissues) at the site of injection.
A diabetic patient
making subcutaneous
injection
subcutaneous injection
in the mouse
This method is particularly useful when
continuous presence of the drug in the tissues
is needed over a long period. The usefulness
of this method is enhanced by the use of
depot preparations from which the drug is
released more slowly than it is from simple
solution e.g. long-acting insulins. Another
form of the depot preparation is
subcutaneous implant.
In this case, a sterile pellet is implanted into
subcutaneous tissue instead of injecting
drug solution e.g. hormones are
administered in this way. If a vasoconstrictor
agent is incorporated in a drug solution, it
retards the absorption e.g. adrenaline is
combined with local anesthetics to prolong
the local anesthesia.
b) Intramuscular:Injection is made deep into the muscle
tissue.
In humans, the best site is deltoid muscle in
the shoulder or the gluteus muscle in the
buttocks. This method is suitable for the
irritating substances that cannot be given by
subcutaneous route. The speed of
absorption from site of injection is
dependent on the vehicle used, absorption is
quick from aqueous solutions and slow from
oily preparations. Absorption is complete,
predictable and faster than subcutaneous
route.
Intramuscular injection in deltoid and gluteal
muscles
Intravenous:Drug solution in injected directly into the
lumen of a vein so that it is diluted in the
venous blood. The drug is carried to the
Heart and circulated to the tissues. Drugs in
oily vehicle or those that cause haemolysis
should not be given by this route. Since the
drug is introduced directly into blood, the
desired concentration of the drug is
achieved immediately which is not possible
by any other procedure.
Intravenous Administration
This route is of prime importance in emergency. Also
certain irritant drugs could be given by this route.
Also this is the only route for giving large volume of
drugs e.g. blood transfusion.
However, there are certain disadvantages of this
procedure.
1. Once the drug is injected nothing can be done to
prevent its action.
2. I/v injection requires technical skill to minimize the
risk of leakage of irritant solution into the surrounding
tissues.
3. Air embolism may cause serious problems.
Intraperitoneal:The peritoneum offers a large absorbing
surface area from which drugs enter
circulation rapidly but primarily by way of
portal vein. Hence First-Pass effect not
avoided.
This is probably the most widely used route
of drug administration in laboratory animals.
In human, it is very rarely employed due to
the dangers of infection and injury to viscera
and blood vessels.
Inraperitoneal Injection
e) Intradermal:Drug are injected into papillary layer of skin.
For example tuberculin injection for
montoux test and BCG vaccination for active
immunization against tuberculosis.
BCG: Bacille Calmette-Guerin
Administering Intradermal Injections:1. Check physician's order
2. Prepare equipment:
Draw up 0.1ml of the medication in a 1cc syringe
- ones called a TB/tuberculin syringe.
Collect medication, procedure gloves, alcohol
wipe, and cotton ball
3. Cleanse the site with an alcohol pad going
in a circular motion
4. Hold the skin taut
5. Hold the needle at a 15 degree angle to the skin
with the bevel facing up.
6. Insert the needle through the skin, just below
the epidermis into the dermis.
7. Inject the fluid, making a bubble just below the
skin
8. Remove the needle
9. Dispose of used syringe and needle in a Sharps
container
Intradermal Injection
f)
Intra Medullary:The needle is introduced into marrow cavity and
effects are similar to those following intravenous
injection. This route is used when veins are not
available specially in children. In adults the
injection is made into marrow cavity of sternum
and under 3 years of age into that of tibia or femur.
g) Intrathecal:Blood brain barrier often prevents the entry of
certain drugs into the central nervous system.
Also the blood CSF barrier prevents the approach of drugs
to the meninges. Thus when local and rapid effects of
drugs on meninges are desired the drugs are injected into
Subarachnoid (between arachnoid mater and pia
mater)space and effects of the drugs are then localized to
the spinal nerves and meninges e.g. intrathecal injection of
streptomycin in tuberculosis and meningitis used to be
used by this route but with the invention of third
generation cephalosporins it is not used any more to treat
these conditions. The injection of local anaesthetics for the
induction of spinal anaesthesia is given by this route.
(the three membranes covering the brain and
spinal cord from outside to inward are dura
mater, arachnoid mater and pia mater)
h) Intra articular:It is also known as intra synovial. Sometimes
drugs are injected into the joint cavity to
localize their action at the site of administration
e.g. Hydrocortisone acetate in the treatment of
rheumatoid arthritis. Local anesthetic is added
to minimize pain of injection. Strict aspesis
must be maintained to avoid joint-infection.
i)
j)
Intra Cardiac:In cardiac arrest intracardiac injection of
adrenaline is made for resuscitation.
Intra-arterial:Sometimes a drug is injected directly into an
artery to localize its effects in a particular
tissue or organ. However, the therapeutic
value of such practice is doubtful.
Infact in human the use of this technique is
restricted to the injection of radio-opaque
media for diagnostic purposes. A competent
person is required to inject the drug intra
arterially. However, there is no fear of firstpass effect when the drug is given by this
route.
3-
INHALATION
Inhalation or Pulmonary Absorption: Gaseous and
volatile drugs may be inhaled. They are then
absorbed by pulmonary endothelium and mucous
membrane of the respiratory tract and reach
circulation rapidly. Volatile or gaseous anaesthetics
such as halothane, enflurane and nitrous oxide are
administered by this route.
Bronchodilators are generally given from
inhalers in aerosol form. Now inhalers have
been developed which allow the supply of
accurately metered doses of drugs. This
development has greatly extended the scope
of this technique.
4 LOCAL OR TOPIOCAL APPLICATION
a) Skin
Drugs applied locally on the skin are poorly
absorbed through the epidermis. However,
dermis is permeable to many solutes. Thus
systemic absorption of drugs occurs more
readily through abraded, burned or denuded
skin. Inflammation and other conditions
that enhance cutaneous blood flow also
promote absorption.
Drugs are applied in the form of ointments,
pastes, poultice and cream to the skin for
their local action. However, absorption
through skin can be increased by suspending
the drug in an oily vehicle and rubbing the
preparation into the skin. This method of
administration is called inunction.
b) Mucous Membranes:Drugs are applied onto the various mucous
membranes for their local action.
i)
Mouth and Pharynx:Bitters are used for their reflex action to improve
digestion. Boroglycerine and gentian violet paint (as
astringent) are used for their effects on buccal mucosa.
ii) Stomach & Intestine:Antacids (to neutralize secreted HCl) and emetics ( to
induce emesis) are used for their local effect
iii) Rectum:-
Drugs are applied in the form of suppository
or enemas e.g. glycerin suppository for their
local action. Drugs are employed for relief of
itching and pain in haemorrhoid.
iv) Respiratory Tract:In infections of respiratory tract, tincture
benzoin co steam inhalations give relief from
nasal congestion, phenyl ephrine nasal drops
are also used for nasal congestion.
v) Vagina:The drugs are used in the form of pessary or
tablet to treat the vaginal infections. Although
this method can be applied for the drugs that
are absorbed through vaginal mucous
membrane into the circulation, it is restricted to
the local treatment of vaginal conditions
vi) Conjunctivae:Mydriatics ( to dilate pupil), miotics (to
constrict the pupil), local anaesthetics
antiseptics and antibiotics are applied to the
conjunctivae for their local action.
Conjunctiva: The delicate membrane lining the
eyelids and covering the eye ball.