Question:

Hi, is the oral form of medicine equivalent to the rectal form of medicine (i.e 500mg of oral table vs 500 mg of rectal tablet)? I notice that there is a medication called Primperan for nausea and vomiting and the oral form is 10 mg per tablet while the rectal form is 20 mg per tablet. This makes me feel that rectal form is not equal to oral form. Please explain. Thanks a lot in advance.

Answer:

The route of administration will often make a difference as to how a medication may be used for therapy. Most medications are ingested orally, but other routes, such as rectal administration, are also used to administer a drug, especially if a patient has trouble swallowing.

Even though the medication may be the same one, the route that is being used to administer it makes a difference as to how much the body will be able to use from it. The absorption and bioavailability (how much is actually used by the body) of a medication plays a large role when talking about equivalence of dosage forms.

In the example described, metoclopramide (Primperan) has an oral bioavailability of 80%, so 80% of the medication is actually absorbed when ingested. The bioavailability of metoclopramide rectal suppositories is 53%, so a lot less is actually absorbed by this route. The absorption of metoclopramide is also delayed with rectal administration. Rectal administration of a 10mg suppository will only achieve a peak blood level of 17-37 ng/ml in 2 hours. A 20mg rectal suppository will achieve a peak blood level of 43-82 ng/ml. Higher doses are needed for metoclopramide suppositories in order to reach therapeutic levels.

This is true for most medications. Rectal absorption is more erratic than absorption following oral administration. The absorption rate is generally slower. Therefore, higher rectal doses, or more frequent administration, may be required to achieve and/or maintain blood concentrations of a medication comparable to those obtained following oral administration.

Submitted by Ashley Tepe, James L. Winkle College of Pharmacy, NetWellness.org PharmD candidate

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