McKenna's Pharmacology for Nursing, 2e

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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. 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.

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: Hydroxocobalamin Indications: Treatment of vitamin B 12 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. deficiency; to meet increased vitamin B 12 requirements related to 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