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Maintenance Dose Calculator

Calculates the dose required to maintain a steady-state plasma concentration.

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Maintenance Dose

Formula first

Overview

The maintenance dose equation is used to determine the mass of drug administered per dosing interval to sustain a specific steady-state plasma concentration. It balances the rate of drug delivery into the systemic circulation against the rate of drug clearance from the body.

Symbols

Variables

MD = Maintenance Dose, C_p = Target Concentration, CL = Clearance, \tau = Dosing Interval, F = Bioavailability

Maintenance Dose
Target Concentration
Clearance
Dosing Interval
Bioavailability

Apply it well

When To Use

When to use: This equation is applied when designing a long-term treatment regimen or adjusting dosages for chronic conditions once a steady state is desired. It assumes the drug follows first-order kinetics, meaning the rate of elimination is directly proportional to the plasma concentration.

Why it matters: Calculating the correct maintenance dose ensures that the drug concentration remains within the therapeutic window, preventing ineffective sub-therapeutic levels or dangerous toxicity. This is especially critical for medications with a narrow therapeutic index, such as digoxin, theophylline, or certain antibiotics.

Avoid these traps

Common Mistakes

  • Failing to convert bioavailability percentage to a decimal (e.g., 90% = 0.9).
  • Using the equation for loading doses, which requires Volume of Distribution (Vd) rather than Clearance.
  • Not adjusting clearance for patients with renal or hepatic impairment.

One free problem

Practice Problem

A physician wants to maintain a steady-state plasma concentration of 15 mg/L for an oral medication. The drug has a clearance of 2 L/h and a bioavailability of 0.5. If the drug is administered every 8 hours, what is the required maintenance dose?

Target Concentration15 mg/L
Clearance2 L/h
Dosing Interval8 h
Bioavailability0.5 0-1

Solve for:

Hint: Multiply the target concentration, clearance, and dosing interval, then divide by the bioavailability.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Goodman & Gilman's The Pharmacological Basis of Therapeutics, 13th Edition
  2. Basic and Clinical Pharmacology, 15th Edition, by Bertram G. Katzung
  3. Applied Biopharmaceutics & Pharmacokinetics, 7th Edition, by Leon Shargel and Andrew B.C. Yu
  4. Wikipedia: Pharmacokinetics
  5. Wikipedia: Maintenance dose
  6. Goodman & Gilman's The Pharmacological Basis of Therapeutics
  7. Applied Biopharmaceutics & Pharmacokinetics (Shargel and Yu)
  8. Basic & Clinical Pharmacology (Katzung)