Notes on Pharmacology

Half life

Half-life refers to the amount of time taken for the body to break down or excrete half of a given dose of drug. For example if a drug has a half life of one hour and 100mg of drug is given this means that in one hour’s time there will be 50mg of that drug left in the patient`s system. In a further one hour’s time after that, that is two hours after the initial dose has been given, there will be 25mg of drug left in the patient. In a further one hour after this time, i.e. three hours after the original dose has been given, there will be 12.5mg of the drug left. One application of this principle is that drugs which have a short half-life will need to be given frequently in order to maintain therapeutic plasma concentrations. Drugs with a longer half-life will need to be given less frequently.

Potentiation

Potentiation of a drug occurs when the effect of that drug is enhanced for some reason. This means that the first drug would have an increased effect for a given dose. Some times, potentiation is caused by administration of two drugs at the same time when a second drug potentates the effect of the first drug. Potentiation may also occur when one drug is given after another drug if some of the given dose of the first drug is still in the patients system at the time of administration of the second drug. For example the effect of ACE inhibitors may be enhanced by any other hypotensive agents and alcohol can enhance the effect of anticoagulants

Routes of administration

There are two possible areas of action for a drug. Firstly a drug may be given for a desired local effect, for example eye or ear drops are intended to work in the eye or the ears. Some creams given topically are intended to work on the skin to which they are applied. Local anaesthetise by definition are given for their local effect. Inhalers are given to have a direct effect on the airways. Secondly a drug may be given to have a systemic effect, (body wide effect). When a drug is given for systemic application the aim is to get the drug into the blood which will transport the drug to all parts of the body.

Drugs may be given for systemic absorption in basically in two ways, enterally and parenterally.

Enterally

When an enteral route is used the drug it is given via the gastro-intestinal tract. This will usually be orally by taking tablets or medicines by mouth, (orally) but it may also include per rectal administration.

Parentally

Parentally means that the drug is given via a route which is not via the gastro-intestinal tract. The following are routes uses for parenteral drug administration with comments on how quickly the drug will start to have an effect.

Subcutaneous injection delivers the drug into the subcutaneous layer, just below the skin. Because this layer is composed mostly of poorly perfused adipose tissue systemic absorption may be slow. Normally a subcutaneous injection would start to work after about 15 - 20 minutes. Absorption would be quicker if the patient is warm as there will be peripheral vasodilatation increasing peripheral blood flow. Conversely when the patient is cold and vasoconstricted the absorption would be slower.

Intra-muscular injection delivers the drug into a skeletal muscle usually the vastus lateralus or the gluteus maximus however occasionally the deltoid may also be used on the upper outer aspect of the arm. Because skeletal muscles are fairly well perfused with blood systemic absorption and effect usually starts in about 5 - 10 minutes.

Intravenous injection delivers the drug directly into the venous blood, this means the onset of the effect of the drug is essentially instantaneous. Intravenous drugs may be given as a bolus injection or as infusions.

Rate of action

When drugs are given intravenously the plasma concentrations rise rapidly to a high level but then fall rapidly. With intramuscular injections the rise in plasma concentrations is slower, the peak concentrations are also lower but the duration of action of the drug is longer. With oral administration the rise in plasma levels is slower still, the peak plasma concentrations are lower than with intramuscular injections but the duration of action is longer.

The dosages of drugs

The dose of a drug refers to the weight of drug given. The metric system is universally used to quantify drug dosages. The smallest unit used in the measurement of drug dose is the microgram (mcg). There are 1 000 mcg in one milligram (mg). There are 1 000 mg in one gram (g). There are 1 000 g in one kilogram (Kg)

To summarise;

1 000 mcg = 1 mg (mcg = microgram)

1 000 mg = 1 g (mg = milligram)

1 000 g = 1 Kg (Kg = kilogram)

Volume

Volume is measured in millilitres (mls). A ml is the volume of fluid in one cubic centimetre (cm³). There are 1 000 mls in a litre.

Concentration

The concentration of a solution is usually expressed in units of mass per units of volume. This is often mg per ml. For example 5 mls of amoxycillin in 5 mls of suspension. Concentration may also be expressed as a percentage, eg. A 10% solution would be 10 g of a solid dissolved in 100 ml of solution. Another way concentration may be described is in moles or millimoles. A millimole is one thousanth of a mole. A mole refers to a specific defined number of molecules.