Prescribing - making up of liquid medications

Liquid medication

Medication in liquid form has a concentration per volume (eg. mg/mL).

I prefer to calculate the concentration per unit volume (usually per mL) in all cases. This can be done simply by dividing the stated concentration.

$[m e d i c a t i o n] = d ose / v o l u m e$
$v = v o l u m e$

With Pamol 250mg/5mL
$[P am o l] = d ose (m g) / v o l u m e (m L)$
$[P am o l] = 250 m g /5 m L$
$[P am o l] = 50 m g / m L$

Therefore, for a 700mg dose
$[P am o l] = 250 m g /5 m L$
$[P am o l] = 50 m g / m L$
$v (d ose) = d ose / co n ce n t r a t i o n$
$v (d ose) = d ose / [P am o l]$
$v (d ose) = 700 m g / (50 m g / m L)$
$v (d ose) = 14 m L$

With paracetamol 120mg/5mL
$[p a r a ce t am o l] = d ose (m g) / v o l u m e (m L)$
$[p a r a ce t am o l] = 120 m g /5 m L$
$[p a r a ce t am o l] = 24 m g / m L$

Therefore, for a 700mg dose
$[p a r a ce t am o l] = 120 m g /5 m L$
$[p a r a ce t am o l] = 24 m g / m L$
$v (d ose) = d ose / co n ce n t r a t i o n$
$v (d ose) = d ose / [p a r a ce t am o l]$
$v (d ose) = 700 m g / (24 m g / m L)$
$v (d ose) = 29.17 m L$

In practice the stated concentration can be used, the calculation takes fewer step but is slightly more complex.

For the same 700mg dose
$[P am o l] = 250 m g /5 m L$
$v (d ose) = d ose / co n ce n t r a t i o n$
$v (d ose) = d ose / [P am o l]$
$v (d ose) = 700 m g / (250 m g /5 m L) = 700 m g * (5 m L /250 m g)$
$∴ v (d ose) = (700 m g /250 m g) * 5 m L = (700 m g * 5 m L) /250 m g$
$v (d ose) = 14 m L$

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Prescribing - making up of liquid medications

Liquid medication