From the Research
No, Benadryl (diphenhydramine) is not equally bioavailable when administered orally versus intravenously. When given intravenously, diphenhydramine has 100% bioavailability because it enters the bloodstream directly. In contrast, oral diphenhydramine has approximately 72% bioavailability, as reported in the study by 1, due to first-pass metabolism in the liver. This means that when taking Benadryl by mouth, a significant portion of the active ingredient is metabolized before reaching systemic circulation. For this reason, oral dosing of diphenhydramine is typically higher than IV dosing to achieve similar therapeutic effects. The standard oral dose for adults is 25-50 mg every 4-6 hours, while IV dosing is often 25 mg administered more slowly. IV administration also produces more rapid onset of action (within minutes) compared to oral administration (30-60 minutes). This difference in bioavailability explains why IV diphenhydramine is preferred in emergency situations like severe allergic reactions when rapid and complete drug delivery is essential.
Some key points to consider:
- The bioavailability of oral diphenhydramine is approximately 72% as reported in the study by 1.
- IV administration of diphenhydramine produces more rapid onset of action compared to oral administration.
- The difference in bioavailability between oral and IV diphenhydramine is due to first-pass metabolism in the liver.
- Oral dosing of diphenhydramine is typically higher than IV dosing to achieve similar therapeutic effects.
It's worth noting that while the study by 1 provides valuable information on the bioavailability of diphenhydramine, other studies such as 2, 3, 4, and 5 do not directly address the question of bioavailability. However, the study by 1 is the most relevant and highest quality study on this topic, and its findings should be given the most weight in determining the bioavailability of diphenhydramine.