Why is the enteral route seem by far the most common way to administer a drug to the body?
Let's review drug administration routes, and see for yourself:
- transdermal/ionophoric: expensive, unreliable release concentrations (which makes it expensive because of the extensive research needed for controlled release), noticeable to others ("why do you have that skin patch, mom?" Er...)
- Sublingual: quite similar to oral route, however absence of first pass hepatic metabolism can result in completely different blood concentrations between the two routes. Impairs speech. Could cause mouth irritation (quite disabling). Requires patient compliance.
- intravenous: quite unpractical in outpatient settings. Invasive measure.
- intramuscular/intradermal: hurts. As all things using a needle. Usable, but impractical in the outpatient setting. Can lead to skin lesions (insulin injection in diabetes, for example)
- rectal: culturally unacceptable to some, unreliable release.
- vaginal: similar to rectal, but only for women obviously
- spinal/epidural: very invasive, only in inpatient settings.
- topical: usually only acts locally, can cause discomfort (eyes)
- oral/enteral: well known pharmacokinetics, mostly invisible to others, practical in the outpatient setting, requires limited patient compliance, well known excipients. Of course there can be secondary effects, but that is the case for all administration routes. Most common side effects are abdominal discomfort and nausea, but no one can see you in the water closet. Plus people are used to see pills as medication, something which might not be the case for other routes, depending on the cultural background of the patient.
This answer is of course somewhat subjective, however the importance of the patient's perception and his/her view of what the administration route implies socially cannot be overemphasized.
Pill, tablet and time release are great enteral treatments because of their ability to control dosage by the patient at home. Compared with an IV and oil/sublinguals they are less effective at due to the greater risk of causing toxic damage to the liver and kidneys but there is little chance of overdose by reason of mechanical failure. That is the reason that viagra and metformin are first lines of defense; followed by insulin and alprostadil if they so fail. You do not infect yourself typically with a pill but with an injection its possible for a needle to be dirty. Likewise also there is the ever fear of an air bubble.