Firstly, as with drug interactions (see Drug Interactions) there are two main types of ADR; pharmacokinetic and pharmacodynamic. These relate to how the body acts on the drug (pharmacokinetic) and how the drug acts on the body (pharmacodynamic).

Pharmacokinetic ADRs

In pharmacokinetic ADRs, any problem the patient may have with drug absorption, distribution, metabolism or excretion may cause an adverse effect of a drug at normal doses in that patient. The two main pharmacokinetic factors that will potentially lead to ADRs relate to the main sites of metabolism and excretion, the liver and kidneys.

Pharmacodynamic ADRs

In pharmacodynamic adverse drug reactions, particular care is needed when drugs, for example, cross the blood brain barrier (BBB). Metoclopramide crosses the BBB and is known to cause dystonia. Domperidone does not cross the BBB so in the patient with Parkinson’s disease choose domperidone or at least avoid metoclopramide.

Type A and Type B adverse drug reactions

ADRs are divided into two main groups Type A and Type B. (Rawlins et al 1977).

Type A (Predictable or augmented)

Type A ADRs are relatively common and are often, discovered during the clinical trials phase of drug development. They are usually related to the pharmacological activity of the drug and occur at normal doses in normal people. They can manifest as additive effects to the original drug effect thus their other name ‘augmented’ ADRs. They are largely predictable so warning of their possibility can be given to patients with guidance as to what to do if they occur. They can cause high levels of morbidity but mortality is low. Type A ADRs might be constipation with opioids due to their action on the gut; dry mouth and blurred vision with tricyclic antidepressants because of their antimuscarinic effects.

Type B (Bizarre)

Type B ADRs, on the other hand are not predictable which leads to their other defining name of ‘bizarre’. When the reason for the ADR is found to be genetic, the ADR is often called a pharmacogenetic reaction. Type B ADRs are allergic reactions or genetic responses.

Types C - E (Chronic, Delayed and End of Use)

It has since been considered necessary by some to further subdivide ADRs into: type C (chronic), where the adverse effect develops after continuous or chronic use such as osteoporosis after long term steroid use; type D or delayed ADRs an example of which was the use of diethylstilboestrol in pregnant mothers to prevent premature labour. In this case teenage daughters born to these mothers were shown to develop vaginal adenocarcinoma significantly more than others. Type E or end of use reactions might include rebound effects after long term use of, for example, nasal decongestants. As a doctor, it is important to remember that you can’t always predict how an individual will react to a new drug so it is important to consider carefully those drug types and specific patient groups (see Dealing with specific medical groups) which are more susceptible to reactions.

European classification of ADRs

European legislation has brought in a simple classification according to incidence of reported ADRs. This terminology is used in the Summary of Product Characteristics (SPC) for drugs as shown on www.medicines.org.uk. These SPCs have all the legally required information about drugs marketed by companies within the Association of the British Pharmaceutical Industry (ABPI), which covers the majority of those manufacturing drugs you will use. The European classification is presented as percentage probabilities of developing an ADR and is shown in table below.