Loading dose

If a drug takes a long time to reach therapeutic levels, then a higher dose (the loading dose) may be given initially before dropping down to a lower maintenance dose.

Drugs which may be started with an initial loading dose include digoxin, teicoplanin, voriconazole and procainamide. Phenytoin for acute status epilepticus should also be given with an initial loading dose, co-administered with a benzodiazepine, to immediately stabilize neuronal membranes and electrical activity during a seizure.

Calculating loading dose
Three variables are used to calculate loading dose:

Cp (peak desired concentration of drug)

Vd (volume of distribution of drug in body)

F (bioavailability)

Loading dose = Cp * Vd / F

For an IV, bioavailability = 1, since the drug is directly introducted to the bloodstream. If the patient requires an oral dose, bioavailability will be less than 1 (depending upon absorption, etc.), requiring a larger loading dose.