Advantages And Disadvantages Of Enteral Administration

By swallowing

For systemic effect Advantages are convenience and acceptability.

Disadvantages are that absorption may be delayed, reduced or even enhanced after food or slow or irregular after drugs that inhibit gut motility (antimuscarinic, opioid). Differences in presystemic elimination are a cause of variation in drug effect between patients. Some drugs are not absorbed (gentamicin) and some drugs are destroyed in the gut (insulin, oxytocin, some penicillins). Tablets taken with too small a quantity of liquid and in the supine position, can lodge in the oesophagus with delayed absorption10 and may even cause ulceration (sustained-release potassium chloride and doxy-cycline tablets), especially in the feeble elderly and those with an enlarged left atrium which impinges on the oesophagus.11

10 A woman's failure to respond to antihypertensive medication was explained when she was observed to choke on drinking. Investigation revealed a large pharyngeal pouch that was full of tablets and capsules. Her blood pressure became easy to control when the pouch was removed. Birch D J, Dehn T C B 1993 British Medical Journal 306:1012.

11 Ideally solid-dose forms should be taken standing up and washed down with 150 ml (tea cup) of water; even sitting (higher intra-abdominal pressure) impairs passage. At least patients should be told to sit and take 3 or 4 mouthfuls of water (a mouthful = 30 ml) or a cupful. Some patients do not even know they should take water.

For effect in the gut Advantages are that the drug is placed at the site of action (neomycin, anthelminthics), and with nonabsorbed drugs the local concentration can be higher than would be safe in the blood.

Disadvantages are that drug distribution may be uneven, and in some diseases of the gut the whole thickness of the wall is affected (severe bacillary dysentery, typhoid) and effective blood concentrations (as well as luminal concentrations) may be needed.

Sublingual or buccal for systemic effect

Advantages are that quick effect is obtained, e.g. with glyceryl trinitrate as an aerosol spray, or as sublingual tablets which can be chewed, giving greater surface area for solution. Spitting out the tablet will terminate the effect.

Disadvantages are the inconvenience if use has to be frequent, irritation of the mucous membrane and excessive salivation which promotes swallowing, so losing the advantages of bypassing presystemic elimination.

Rectal administration

For systemic effect (suppositories or solutions).

The rectal mucosa has a rich blood and lymph supply and, in general, dose requirements are either the same or slightly greater than those needed for oral use. Drugs chiefly enter the portal system, but those that are subject to hepatic first-pass elimination may escape this if they are absorbed from the lower rectum which drains directly to the systemic circulation. The degree of presystemic elimination thus depends on distribution within the rectum and this is somewhat unpredictable.

Advantages are that a drug that is irritant to the stomach can be given by suppository (aminophylline, indomethacin); the route is suitable in vomiting, motion sickness, migraine or when a patient cannot swallow, and when cooperation is lacking (sedation in children).

Disadvantages are psychological in that the patient may be embarrassed or may like the route too much; rectal inflammation may occur with repeated use and absorption can be unreliable, especially if the rectum is full of faeces.

For local effect, e.g. in proctitis or colitis, an obvious use.

A survey in the UK showed that a substantial proportion of patients did not remove the wrapper before inserting the suppository.

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