Administering medication using the correct route

Medicines are administered by various routes either directly into or onto the body 

Two main factors determine whether or not a drug will reach its intended site of action in the body:

  • The bioavailability of the drug;
  • How the drug is given (route of administration).

Let's consider the routes:

  • Oral
  • Sublingual
  • Rectal
  • Topical
  • Parenteral – Intravenous, intramuscular, subcutaneous

Oral administration

This is the most frequently used route of drug administration and is the most convenient and economic. The oral route is nevertheless problematic because of the unpredictable nature of gastro-intestinal drug absorption.

For patients, their main advantage is that doses usually only need to be taken once or twice daily. Damage to the release controlling mechanism, for example by chewing or crushing, can result in the full dose of drug being released at once rather than over a number of hours. This may then be absorbed leading to toxicity or may not be absorbed at all leading to sub optimal treatment.

Nurses should seek advice from a pharmacist or the prescribing doctor if they are uncertain about a formulation of solid dose forms and whether or not they are suitable for crushing.

Sublingual

The sublingual mucosa offers a rich supply of blood vessels through which drugs can be absorbed. This is not a common route of administration but it offers rapid absorption into the systemic circulation. The most common example of sublingual administration is glyceryl trinitrate in the treatment of acute angina.

The pharmaceutical industry has formulated and marketed ‘wafer’-based versions of tablets that dissolve rapidly under the tongue. These are aimed at particular markets where taking tablets may be problematic, such as the treatment of migraine (rizatriptan) where symptoms of nausea may deter patients from taking oral treatments. The formulation is also used to treat conditions where compliance with prescribed drug regimens may be problematic, for example, olanzapine used to treat schizophrenia can be administered by the sublingual route.

Rectal administration

The rectal route has considerable disadvantages in terms of patient acceptability (in the UK at least) and unpredictable drug absorption but it does offer a number of benefits. It offers a valuable means of localised drug delivery into the large bowel, for example the use of rectal steroids in the form of enemas or suppositories in the treatment of inflammatory bowel disease. Antiemetics can be administered rectally for nausea and vomiting and paracetamol can be give to treat patients with a pyrexia who are unable to swallow.

Topical administration

The topical application of medicines has obvious advantages in the management of localised disease. The drug can be made available almost directly at the intended site of action, and because the systemic circulation is not reached in great concentration, the risk of systemic side-effects is reduced. For example:

  • The use of eye drops containing beta blockers in the treatment of glaucoma;
  • The application of topical steroids in the management of dermatitis;
  • The use of inhaled bronchodilators in the treatment of asthma;
  • The insertion of pessaries containing clotrimazole in the treatment of vaginal candidiasis.

Topical administration has also become a popular way of introducing drugs into the systemic circulation through the skin. The development of transdermal patches that contain drugs began with the introduction of a hyoscine-based product for the treatment of nausea in the early 1980s.

The market for such products has since grown to include a wide range of disease management areas including the prophylaxis of angina (glyceryl trinitrate), the treatment of chronic pain (fentanyl) and hormone replacement (oestrogens). While the use of transdermal drug administration is not without its problems - for example, some preparations can cause local skin reactions - many patients find it a welcome alternative to taking tablets.

Parenteral administration

Parenteral drug administration can be taken literally to mean any non-oral means of drug administration, but it is generally interpreted as relating to injection directly into the body, by-passing the skin and mucous membranes. The common routes of parenteral administration are intramuscular (IM), subcutaneous and IV.

Advantages of parenteral administration:

  • Drugs that are poorly absorbed, inactive or ineffective if given orally can be given by this route
  • The intravenous route provides immediate onset of action
  • The intramuscular and subcutaneous routes can be used to achieve slow or delayed onset of action
  • Patient compliance problems are largely avoided .

Disadvantages of parenteral administration:

  • Requires trained staff to administer
  • Can be costly
  • Can be painful
  • Aseptic technique is required
  • May require supporting equipment for example, programmable infusion devices

Bioavailability

Bioavailability is the proportion of an administered drug that reaches the systemic circulation and is therefore available for distribution to the intended site of action.

Drugs that are given by direct IV injection are said to have 100% bioavailability. Some drugs that are particularly well absorbed by the gastrointestinal mucosa may have bioavailability comparable to that of an IV dose – for example the antibiotic ciprofloxacin. Most drugs do not have this availability by the oral route so the dose given orally is usually higher than that given parenterally. For example, the beta-blocker propranolol when given orally is administered in doses of 40mg and above. The equivalent IV dose is 1mg.The route of administration and its formulation (tablet, capsule, liquid) can clearly influence the bioavailability of a drug.