Roman numerals are used in writing prescriptions. They
are used to specify the amounts of ingredients when the apothecary
system is being used. They are used to specify the number of units
(capsules, tablets, powders, suppositories, and so forth) to be
dispensed; for example, "Dispxxiv." And lastly, they are used in the
signa or directions to the client. You should, therefore, be
thoroughly familiar with the system of Roman numerals used in
pharmacy. The basic symbols or numerals are:

ss, 1/2

I 1

V 5

X 10

L 50

C 100

D 500

M 1000

These basic numerals may be combined to represent any
number, and there are definite rules for the manner in which they are
combined. Upper-case or lower-case letters may be used for Roman
numerals. Prescribers usually prefer uppercase letters, but they dot
the "I" for the sake of clarity. The rules for Roman numerals are as
follows:

a. Fractions. Except
for "ss" meaning one-half (1/2), all other fractions are represented
by Arabic numerals (1/4, 3/8, 1/120, and so forth). (Note: The "ss"
may be written with or without a bar--ss or .)

b. Repeating Numerals.
Numerals may be repeated. When they are, the value of the number is
repeated. Thus, iii or III is 3 (1+1+1), XXX is 30 (10+10+10), and CCC
is 300 (100+100+100). Any numeral that would be the same as another
when repeated is NOT repeated. For example, VV is NOT used for 10
(5+5) because X is 10 and LL is NOT used for 100 (50+50) because C =
100).

c. Smaller Numerals Before
Larger. When a smaller numeral placed before a larger one, the
smaller value is subtracted from the larger one. Only one number can
be subtracted in this way. Thus, IV (5 - 1) = 4; IX (10 - 1) = 9; and
XC (100 - 10) = 90 are correct, but 3 is never written IIV.

d. Smaller Numerals After
Larger. A smaller numeral placed after a larger one is added to
the larger number. For example, VIII = (5+3) = 8 ; XIII = (10+3) = 13;
CLX = (100+50+10) = 160.

e. Smaller Numeral Between Two
Larger. A smaller numeral between two larger ones is ALWAYS
subtracted from the larger numeral which follows it as CXL (100 +
[50-10]) = 140; MCMLXXVI (1000 + [1000-100] + 50 + 10 + 10 + 5 + 1) =
1976.

f. The Use of "j." As a
precaution against error, the last "i" may be replaced by a "j." When
this method is used, 3 would be written as iij.

g. Table of Roman Numerals.
Table 1-6 shows examples of Roman numerals and their equivalents.

h. Number After Modified Noun.
When a number expressed in Roman numerals is used to modify a noun,
the number follows the noun. The noun is likely to be a unit of the
apothecary system or a unit of dosage. For example, "gr ii" would be
interpreted as "two grains" and "caps i" would mean "one capsule."

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From Drug
Dosage and Therapy