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P A R T 1
Introduction to nursing pharmacology
give for the ordered dose. In order to calculate this, we
use the following formula:
Number of tablets or capsules =
Strength required
Strength in stock
Try an example: An order is written for 0.05 g
Aldactone
(spironolactone) to be given orally (PO). The
Aldactone
is available in 25 mg tablets. How many
tablets would you have to give? First, you will need to
convert the grams to milligrams:
So 0.05 g of
Aldactone
is equal to 50 mg of
Aldactone
.
The order has been converted to the same measure-
ment as the available tablets. Now solve for the number
of tablets that you will need:
Strength required
Strength in stock
=
50 mg
25 mg
= 2 tablets
Sometimes the desired dose will be a fraction of
a tablet or capsule,
1
/
2
or
1
/
4
. Some tablets come with
scored markings that allow them to be cut. Pill cutters
are readily available in most pharmacies to help people
cut tablets appropriately. However, one must use caution
when advising a person to cut a tablet. Many tablets
come in a matrix system that allows for slow and steady
release of the active drug. These drugs cannot be cut,
crushed or chewed. Always consult a drug reference
before cutting a tablet. However, as a quick reference,
any tablet that is designated as having delayed or sus-
tained release may very well be one that cannot be cut.
Capsules can be very difficult to divide precisely, and
some of them also come with warnings that they cannot
be cut, crushed or chewed. If the only way to deliver
the correct dose to a person is by cutting one of these
preparations, a different formulation of the drug, a dif-
ferent drug or a different approach to treating the person
should be tried.
Other oral drugs come in liquid preparations. Many
of the drugs used in paediatrics and for adults who
might have difficulty swallowing a pill or tablet are
prepared in a liquid form. Some drugs that do not come
in a standard liquid form can be prepared as a liquid
by the pharmacist. If the person is not able to swallow
a tablet or capsule, check for other available forms
and consult with the pharmacist about the possibility
of preparing the drug in a liquid as a suspension or a
solution. The formula for calculating liquid volumes is
as follows:
Strength required
Strength in stock
×
volume of stock solution
Try this example: An order has been written for
125 mg of phenytoin. The bottle states that the solution
contains 250 mg/5 mL. How much of the liquid should
you give?
Strength required
Strength in stock
×
volume of stock solution
=
125
250
×
5 =
625
250
= 2.5 mL
Even if you are working in an institution that
provides unit-dose medications, practise your calcu-
lation skills to keep them sharp. Power can be lost,
computers can go down and the ability to determine cal-
culations is a skill that anyone who administers drugs
should have in reserve. Periodically throughout this text
you will find a Focus on Calculations box to help you
refresh your dose calculation skills as they apply to the
drugs being discussed.
Parenteral drugs
All drugs administered parenterally must be adminis-
tered in liquid form. The person administering the drug
needs to calculate the volume of the liquid that must
be given to administer the prescribed dose. The same
formula can be used for this determination that was
used for determining the dose of an oral liquid drug:
Strength required
Strength in stock
×
volume of stock solution
= amount of drug to administer
Try this example: An order has been written for
75 mg of pethidine to be given intramuscularly (IM).
The vial states that it contains pethidine 50 mg in
1.0 mL. Set up the equation just as before:
75 mg
50 mg
×
1 mL
=
75
/
50
=
1.5 mL to be administered
Intravenous solutions
Intravenous (IV) solutions are used to deliver a pre-
scribed amount of fluid, electrolytes, vitamins, nutrients
or drugs directly into the bloodstream. For infusions,
most institutions now use electronically monitored
delivery systems. However, it is still important to be
able to determine the amount of an IV solution that
should be given, using standard calculations. Most
IV delivery systems come with a standard control,
by which each millilitre delivered contains 20 drops.
Microdrip systems, which usually deliver 60 drops/mL,
are also available; they are usually used in paediatric
settings. Always check the packaging of the IV tubing
to see how many drops/mL are delivered by that par-
ticular device if you have any doubts or are unfamiliar
with the system.
