788
P A R T 8
Drugs acting on the cardiovascular system
in RBCs (see Figure 49.3). Vitamin B
12
is also necessary
for maintenance of the myelin sheath in nerve tissue.
Both are given as replacement therapy for dietary defi-
ciencies, as replacement in high-demand states such
as pregnancy and breastfeeding, and to treat megalo-
blastic anaemia. Folic acid is used as a rescue drug for
cells exposed to some toxic chemotherapeutic agents.
Calcium folinate is used as a rescue drug following
methotrexate therapy to decrease the toxicity of metho
trexate caused by decreased elimination or overdose of
folic acid antagonists such as trimethoprim and for the
treatment of various megaloblastic anaemias.
Pharmacokinetics
Folic acid can be given in oral, intramuscular, intra
venous and subcutaneous forms. The parenteral drugs
are preferred for people with potential absorption
problems; others should be given the oral form if at all
possible. Calcium folinate is a reduced form of folic acid
that is available for oral, intramuscular and intravenous
use.
Hydroxocobalamin must be given intramuscularly
every day for 5 to 10 days to build up levels, then once
a month for life. It cannot be taken orally because the
problem with pernicious anaemia is the inability to
absorb vitamin B
12
secondary to low levels of intrinsic
factor. It can be used in states of increased demand (e.g.
pregnancy, growth spurts) or dietary deficiency, but oral
vitamins are preferred in most of those cases. Cyanoco-
balamin is not as tightly bound to proteins and does not
last in the body as long as hydroxocobalamin does. This
drug is primarily stored in the liver and slowly released
as needed for metabolic functions.
Folic acid and vitamin B
12
are well absorbed after
injection, metabolised mainly in the liver and excreted
in urine. These vitamins are considered essential during
pregnancy and breastfeeding because of the increased
demands of the mother’s metabolism.
Contraindications and cautions
These drugs are contraindicated in the presence of
known allergies to these drugs or to their components
to avoid hypersensitivity reactions
. They should be used
cautiously in people who are pregnant or breastfeeding
or who have other anaemias
to ensure that the correct
doses of the drug are used to provide the best therapeu-
tic effect and decrease the risk of toxic effects
.
Adverse effects
These drugs have relatively few adverse effects because
they are used as replacement for required chemicals.
Hydroxocobalamin has been associated with itching,
rash and signs of excessive vitamin B levels, which can
also include peripheral oedema and heart failure. Mild
diarrhoea has been reported with these drugs. Pain and
discomfort can occur at injection sites. Nasal irritation
can occur with the use of intranasal spray.
Prototype summary: Folic acid
Indications:
Treatment of megaloblastic anaemia due
to sprue, nutritional deficiency.
Actions:
Reduced form of folic acid, required for
nucleoprotein synthesis and maintenance of normal
erythropoiesis.
Pharmacokinetics:
Route
Onset
Peak
Oral, IM, SC, IV Varies
30–60 mins
T
1/2
:
Unknown; metabolised in the liver and excreted
in urine.
Adverse effects:
Allergic reactions, pain and
discomfort at injection site.
Prototype summary: Hydroxocobalamin
Indications:
Treatment of vitamin B
12
deficiency; to
meet increased vitamin B
12
requirements related to
disease, pregnancy or blood loss.
Actions:
Essential for nucleic acid and protein
synthesis; used for growth, cell reproduction,
haematopoiesis, and nucleoprotein and myelin
synthesis.
Pharmacokinetics:
Route
Onset
Peak
IM
Intermediate 60 mins
T
1/2
:
24–36 hours; metabolised in the liver and
excreted in urine.
Adverse effects:
Itching, transitory exanthema,
mild diarrhoea, anaphylactic reaction, heart
failure, pulmonary oedema, hypokalaemia, pain at
injection site.
Care considerations for people receiving
folic acid derivatives or vitamin B
12
Assessment: History and examination
■
■
Assess
for contraindications or cautions
:
any known allergies to these drugs or drug
components, other anaemias, pregnancy,
breastfeeding and nasal erosion.
■
■
Assess baseline status before beginning therapy
to determine any potential adverse effects.
This
includes affect, orientation and reflexes; pulse, blood
pressure and perfusion; respirations and adventitious
