4.1 When is intravenous therapy appropriate?
Intravenous therapy may be the most appropriate option when:
- High plasma levels of a drug are required rapidly. Unlike other routes, the drug is introduced directly into the bloodstream and is available to exert its pharmacological effect as soon as it enters the body. Medicines given by other routes need to be absorbed into the bloodstream first, which can take considerable time. Oral medicines are usually absorbed from the small intestine, while medicines administered intramuscularly must be absorbed from muscle fibres into the bloodstream. The intravenous route is usually the route of choice in emergencies because it is usually the fastest way to achieve a therapeutic effect.
- Tight control of drug levels is required, with the need for small adjustments to the rate of administration, according to the patient’s response. This can be achieved by giving the drug as a continuous infusion. Examples of such infusions include insulin for blood glucose control and the infusion of anaesthetic agents during surgery to maintain unconsciousness.
- Patients are unable to take oral medication. This may be because they are vomiting or unconscious, or because they have had recent gastrointestinal surgery.
- Patients are unable to absorb medicine orally, for example those who have severe diarrhoea, active Crohn’s or coeliac disease.
- Rapid correction of fluid or electrolytes is required, for example after haemorrhage.
- Other routes are not available. For example, the intramuscular route may not be appropriate in the very young or the very old as they tend to have a reduced muscle mass, which is not ideal for the administration of medicines. Those receiving anticoagulant medicines or patients with clotting diseases such as haemophilia may bleed from the IM injection site.
- Other routes are not acceptable to the patient. IM injections can be painful, and may be refused, even by healthy individuals. Many UK patients refuse suppositories.
4.2 Drug factors that influence the choice of route
Some medicines must be given by the intravenous route because of their chemical or pharmacological properties.
4.2.1 Absorption
Some drugs are broken down by gastric secretions, which prevents them from being given orally. Proteins such as insulin and infliximab are inactivated in the gut so must be injected. Other drugs do not possess the chemical properties to cross the gut wall so cannot be given orally to cause a systemic effect. However, these drugs may still be useful for treating diseases of the gastrointestinal tract, e.g. vancomycin cannot be given orally to treat a systemic infection as it is not absorbed, but can be used to treat Clostridium difficile infection of the intestine.
Some drugs may be given by subcutaneous, intramuscular or rectal routes, but the absorption from these sites may be erratic and unreliable. Gentamicin may be given by IM injection, but to treat serious infection the intravenous route is used in preference as therapeutic levels are more likely to be achieved.
4.2.2 The first-pass effect
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