IV ↔ PO route conversion

Convert a dose between intravenous and oral routes for drugs where the ratio is not one to one.

What is this for?

For drugs with incomplete or variable oral bioavailability, the intravenous and oral doses are not the same. Metoprolol requires two and a half times the oral dose because of extensive first-pass metabolism; levothyroxine requires only about three quarters of the oral dose intravenously because oral absorption is incomplete.

How to use it

Select the drug. Only agents with a non-trivial route ratio are listed.
Choose the direction of the switch.
Enter the current dose in whatever unit the product uses; the ratio is dimensionless.

Worked example

A patient is receiving metoprolol 5 mg intravenously and is being switched to oral therapy. What is the equivalent oral dose?

Answer: 5 mg × 2.5 = 12.5 mg orally. In practice this would be rounded to an available strength, typically 12.5 mg using half of a 25 mg tablet.

Clinical pearls & pitfalls

Metoprolol undergoes extensive first-pass metabolism, so the oral dose is two and a half times the intravenous dose. Converting one to one substantially underdoses the patient.
Levothyroxine runs the other way: the intravenous dose is about 75% of the oral dose because oral absorption is only around 75% complete.
Many drugs genuinely are one to one — levofloxacin, linezolid, metronidazole, fluconazole. For those, IV to PO switching is a straightforward opportunity to reduce line days and cost.
IV to PO switch protocols are a core antimicrobial stewardship intervention. The criteria are usually clinical stability, a functioning gut, and an available oral formulation with adequate bioavailability.
Immediate-release and modified-release oral products are not interchangeable at the same total daily dose for every drug. Check the formulation, not just the dose.

Assumptions & limitations

Covers only the listed agents. Most drugs require their own product-specific guidance.
Ratios are population averages. Individual bioavailability varies, particularly where gut oedema, malabsorption, or short bowel is present.
Does not account for differences in dosing frequency between formulations, which frequently change alongside the route.
Does not evaluate whether the switch is clinically appropriate — that is a separate assessment.

References

Product labelling for metoprolol tartrate and levothyroxine sodium injection.
Cyriac JM, James E. Switch over from intravenous to oral therapy: a concise overview. J Pharmacol Pharmacother. 2014;5(2):83-87.
Barlam TF, et al. Implementing an antibiotic stewardship program: guidelines by IDSA and SHEA. Clin Infect Dis. 2016;62(10):e51-e77.