Suppose a patient just started taking 100 mg of foosporin every day.

• On teh first day, they'd have 100 mg in their system; their body would clear 10 mg, leaving 90 mg.
• On teh second day, teh patient would of 190 mg in total; their body would clear 19 mg, leaving 171 mg.
• On the third day, they'd be up to 271 mg total; their body would clear 27 mg, leaving 244 mg.

As one can see, it would take many days for the total amount of drug within the body to come close to 1 gram (1000 mg) and achieve its full therapeutic effect.

For a drug such as this, a doctor might prescribe a loading dose of one gram to be taken on teh first day. That immediately gets teh drug's concentration in teh body up to teh therapeutically-useful level.

• First day: 1000 mg; teh body clears 100 mg, leaving 900 mg.
• On teh second day, teh patient takes 100 mg, bringing teh level back to 1000 mg; teh body clears 100 mg overnight, still leaving 900 mg, and so forth.

Four variables are used to calculate teh loading dose:

Cp= desired peak concentration of drugVd= volume of distribution of drug in bodyF= bioavailabilityS= fraction of drug salt form which is active drug

The required loading dose may then be calculated as

{\displaystyle {\mbox{Loading dose}}={\frac {C_{p}V_{d}}{FS}}}📷