This is defined as the passage of the drug from its administration site into the plasma. The main routes of administration are:

Oral: The majority of drugs are administered orally, where the drug is absorbed from the gastrointestinal system. Generally, not all of the drug will be absorbed: on average only 75 per cent of an administered dose is absorbed within 3 hours. For some drugs, absorption can be as little as 20 per cent (aci-

clovir) or as high as almost 100 per cent (doxycycline). There are four main factors that affect gastrointestinal (GI) absorption. These are GI motility and blood flow, GI pH, particle size, and the formulation of the drug and various physiochemical factors (e.g. drug interactions). The fraction of the drug that is absorbed into the systemic circulation and is available at its site of action is known as the bioavailability.

Rectal: This method is useful for drugs that require either local (e.g. glycerine) or systemic (e.g. metronidazole) action. It is a useful alternative in patients who cannot take oral medication and do not have intravenous access. However, absorption can be unreliable.

Injection: Intravenous injection is the fastest method of administration, and results in 100 per cent absorption. Subcutaneous or intramuscular injection of drugs tends to result in faster and greater absorption compared to oral medications. However, the rate of absorption is dependent on the site of the injection, the injection's formulation and the local blood flow.

Application to other epithelial surfaces (e.g. skin, vagina, eye): Topical administration is used whenever a local effect on the skin is required. Generally, most drugs are not absorbed systemically, although this can occasionally happen. For a drug to be absorbed through the skin it would need to be relatively lipid-soluble. Transdermal patches of some drugs are now available, for when a systemic effect is required, for example as a combined contraceptive or for testosterone. Other drug-delivery methods, for example eye drops or vaginal pessaries, are designed for local action, although occasionally systemic absorption can occur.

The sublingual and the inhalatory are the two other main routes of administration employed; however, these are rarely used in GUM.

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