Most drugs can be administered by a variety of routes .The choice appropriate route in a given situation depend both on drug as well as patient related factors. Mostly common sense consideration ,feasibility ,and convenience dictate the route to be used .
Thus, high concentrations are attained are the desired sites without exposing the rest of the body. Systemic side effect or toxicity are consequently absent or minimal. for drugs (in suitable dosage forms) that are absorbed from these sites /routes ,the same can serve as systemic route of administration e.g. glyceral trinitrates (GTN) applied on the skin as ointment or transdermal patch. The local routes are :-
The drug administered through systemic routes is intended to be absorbed into the blood stream and distributed all over ,including the site of action, through circulation .
Oral ingestion is the oldest and commonest mode of drug administration .It is safer ,more convenient ,does not need assistent noninvasive, often painless , medicament need not to be sterile and so is cheaper .Both solid dosage forms (powders, tablets, capsules, spansules, dragees, moulded tablets, gastrointestinal therapeutic systems—GITs) and liquid dosage forms (elixirs, syrups, emulsions, mixtures) can be given orally.
Limitations of oral route of administration
• Action of drugs is slower and thus not suitable for emergencies.
• Unpalatable drugs (chloramphenicol) are difficult to administer; drug may be filled in capsules to circumvent this.
• May cause nausea and vomiting (emetine).
• Cannot be used for uncooperative/unconscious/ vomiting patient.
• Absorption of drugs may be variable and erratic; certain drugs are not absorbed (streptomycin).
• Others are destroyed by digestive juices (penicillin G, insulin) or in liver (GTN, testosterone, lidocain e).
- Sublingual (s.l.) or buccal
The tablet or pellet containing the drug is placed under the tongue or crushed in the mouth and spread over the buccal mucosa. Only lipid soluble and non-irritating drugs can be so administered.
Absorption is relatively rapid—action can be produced in minutes. Though it is somewhat inconvenient, one can spit the drug after the desired effect has been obtained. The chief advantage is that liver is bypassed and drugs with high first pass metabolism can be absorbed directly into systemic circulation. Drugs given sublingually are—GTN, buprenorphine, desaminooxytocin.
Certain irritant and unpleasant drugs can be put into rectum as suppositories or retention enema for systemic effect. This route can also be used when the patient is having recurrent vomiting or is unconscious. However, it is rather inconvenient and embarrassing; absorption is slower, irregular and often unpredictable, though diazepam solution is rapidly and dependably absorbed from rectum in children.
Highly lipid soluble drugs can be applied over the skin for slow and prolonged absorption. The liver is also bypassed. The drug can be incorporated in an ointment and applied over specified area of skin. Absorption of the drug can be enhanced by rubbing the preparation, by using an oily base and by an occlusive dressing.
Volatile liquids and gases are given by inhalation for systemic action, e.g. general anaesthetics. Absorption takes place from the vast surface of alveoli—action is very rapid. When administration is discontinued the drug diffuses back and is rapidly eliminated in expired air. Thus, controlled administration is possible with moment to moment adjustment. Irritant vapours (ether) cause inflammation of respiratory tract and increase secretion.
The mucous membrane of the nose can readily absorb many drugs; digestive juices and liver are bypassed. However, only certain drugs like GnRH agonists and desmopressin applied as a spray or nebulized solution have been used by this route. This route is being tried for some other peptide drugs, like insulin.
- Parenteral (Par—beyond, enteral—intestinal)
This refers to administration by injection which takes the drug directly into the tissue fluid or
blood without having to cross the intestinal mucosa. The limitations of oral administration
are circumvented.
Drug action is faster and surer (valuable in emergencies). Gastric irritation and vomiting are not provoked. Parenteral routes can be employed even in unconscious, uncooperative or vomiting patient. There are no chances of interference by food or digestive juices. Liver is bypassed.
Disadvantages of parenteral routes are—the preparation has to be sterilized and is costlier, the technique is invasive and painful, assistance of another person is mostly needed (though self injection is possible, e.g. insulin by diabetics), there are chances of local tissue injury and, in general, parenteral route is more risky than oral.
The important parenteral routes are:
The drug is deposited in the loose subcutaneous tissue which is richly supplied by nerves (irritant drugs cannot be injected) but is less vascular (absorption is slower than intramuscular). Only small volumes can be injected s.c. Self-injection is possible because deep penetration is not needed.
The drug is injected in one of the large skeletal muscles—deltoid, triceps, gluteus maximus, rectus femoris, etc. Muscle is less richly supplied with sensory nerves (mild irritants can be injected) and is more vascular (absorption of drugs in aqueous solution is faster).
It is less painful, but self injection is often impracticable because deep penetration is needed. Depot preparations (oily solutions, aqueous suspensions) can be injected by this route.
Intramuscular injections should be avoided in anticoagulant treated patients, because it can produce local
haematoma.
The drug is injected as a bolus (Greek: bolos–lump) or infused slowly over hours in one of the superficial veins. The drug reaches directly into the blood stream and effects are produced immediately (great value in emergency). The intima of veins is insensitive and drug gets diluted with blood, therefore, even highly irritant drugs can be injected i.v., but hazards are—
thrombophlebitis of the injected vein and necrosis of adjoining tissues if extravasation occurs. These complications can be minimized by diluting the drug or injecting it into a running i.v. line. Only aqueous solutions (not suspensions) can be injected i.v. and there are no depot preparations for this route.
The dose of the drug required is smallest (bioavailability is 100%) and even large volumes can be infused. One big advantage with this route is—in case response is accurately measurable (e.g. BP) and the drug short acting (e.g.
sodium nitroprusside), titration of the dose with the response is possible. However, this is the most risky route—vital organs like heart, brain, etc. get exposed to high concentrations of the drug.
The drug is injected into the skin raising a bleb (e.g. BCG vaccine, sensitivity testing) or scarring/multiple puncture of the epidermis through a drop of the drug is done. This route is employed for specific purposes only.
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