OTHER ROUTES OF ADMINISTRATION

When a compound is administered by topical application, the target is normally, but not always (e.g. a nicotine patch), local as with local anaesthetics. There is a fatty protective barrier on the skin that the substance has to traverse. The substance may therefore be applied in a solvent or as a cream that helps it to cross this barrier. A fat-solubilizing group such as ester may be chemically attached to the drug. This group may be removed subsequently by esterases within the body to reveal the active drug once it has crossed the barrier. The esterification of cortical steroids that are used in creams to alleviate skin conditions such as psoriasis, exemplifies this. Intravenous injection provides a rapid route enabling a compound to enter the circulatory system without having to pass the liver and be subject to ‘first pass loss’. While the onset of action from intravenous injection is quite rapid, that of intramuscular action is slower and the time-scale over which the drug acts is longer. Injection into the correct site is a skilled method of administration and its use is therefore restricted. The final major method of administration, inhalation, is rapid and it is not restricted to compounds that are volatile. Indeed, a number of nonvolatile compounds such as steroids that are used in the treatment of asthma are given by inhalation as aerosols. The advantage of inhalation in the treatment of asthma is the directness of action.