Loading Dose
Initial higher dose to rapidly achieve the desired plasma concentration.
Formula
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Example
Ctarget = 15 mg/L, Vd = 40 L, F = 0.8 → LD = 750 mg
How it works
A loading dose is used when you need a drug level in the blood to rise quickly instead of waiting for many half-lives of maintenance dosing. The formula says the amount needed depends on the target concentration and the apparent space the drug distributes into (Vd), then it is adjusted for bioavailability because only a fraction of an oral dose may actually reach the bloodstream. In other words, you must give enough drug to fill the body’s distribution space up to the desired plasma concentration.
When to use it
Use this when starting a medicine that needs a rapid therapeutic effect, especially if the drug has a long half-life or the patient needs prompt control of symptoms. It is common for drugs given by mouth or other non-IV routes where bioavailability is less than 100%.
Worked example, step by step
Step 1: Write the formula: LD = (C_target × V_d) / F Step 2: Substitute the values: LD = (8 mg/L × 50 L) / 0.5 Step 3: Multiply the top: 8 × 50 = 400 mg Step 4: Divide by bioavailability: 400 / 0.5 = 800 mg Final answer: LD = 800 mg
Common mistakes to avoid
- Forgetting to divide by bioavailability when the drug is not given intravenously
- Using the maintenance dose formula instead of the loading dose formula
- Mixing units, such as using concentration in mg/mL with Vd in L without converting consistently
Did You Know?
Digoxin and many antibiotics require loading doses because waiting for steady state would take dangerously long.
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More in Pharmacology & Dosage
View allDrug Half-Life
IntermediateTime for the plasma concentration of a drug to fall to half its initial value.
Steady-State Concentration
IntermediateAverage plasma drug concentration when intake and elimination are balanced.
Maintenance Dose
IntermediateRecurring dose needed to sustain the steady-state concentration.
Volume of Distribution
BasicApparent volume into which a drug distributes — a larger Vd means the drug leaves the bloodstream and enters tissues.
Clearance
IntermediateVolume of plasma completely cleared of a drug per unit time.
Bioavailability
IntermediateFraction of an oral dose that reaches systemic circulation compared with an IV dose.