Virtual Medical Centre

Adverse Drug Reactions

• Adverse Drug Reaction
• Definitions
• Classification of Adverse Drug Reactions
• Host Factors Influencing Adverse Drug Reactions
• Drug Factors Influencing Adverse Drug Reactions
• Diagnosis of an Adverse Drug Reaction
  • Clinical History
  • Laboratory Investigations
• Treating an Adverse Drug Reaction

Adverse Drug Reactions

Adverse events following medical therapies are common. The term 'adverse reaction' is preferred by doctors over the more commonly used term 'side effect'. This is because side effect implies that the effects are not explained by the pharmacological actions of the drug. Studies have shown that between 5% and 30% of all patients receiving medical therapies develop an adverse event. In many cases, it is not known what the underlying mechanism is for these adverse events, thus it is difficult to prevent recurrence in the future. This article will discuss what is known about adverse drug events and outline an approach for investigating them and predicting the likelihood of a recurrence. 

Adverse Drug Reaction Definitions

Adverse drug reaction (ADR) is a term used to describe the unwanted, negative consequences of drug therapy. Our immune system is a set of mechanisms that work to protect us from infection by identifying and destroying pathogens. The term drug hypersensitivity is used to describe any drug reaction that our immune system mounts unnecessarily. Drug allergy is restricted specifically to a reaction that occurs when special group of antibodies (Ig E) are activated. These antibodies are large, Y shaped proteins used by the immune system to recognise and destroy foreign bodies such as bacteria and viruses. (Learn more about the immune system.)

Classification of Adverse Drug Reactions

There are many ways of classifying ADRs - these are outlined in Table 1. The causes of ADRs can be divided into those mediated by our immune system and those that aren't. The majority (75-80%) are type A (ie pharmacologically mediated) and are caused by non-immunologic effects. Immune mediated reactions account for 5-10% of all ADRs and are generally classified from Type 1 to Type 4, depending on the immune mechanism involved (see Table 2). Pseudoallergic or anaphylactoid reactions (ie reactions resembling immediate hypersensitivity reactions but are not mediated by allergen-IgE interaction) occur when a particular type of cell - the mast cell, (responsible primarily for becoming activated in allergy and anaphylaxis), is activated. Drugs such as opiates (a group of pain medications), vancomycin (a type of antibiotic) and radiocontrast media can trigger these reactions. These reactions may be hard to tell apart from true Ig E mediated reactions (also known as Type 1 sensitivity). In addition there are other specific drug hypersensitivity syndromes caused by non-IgE immune mechanisms (see Table 3).

Host Factors Influencing Adeverse Drug Reactions

Adverse reactions are known to be more common in older persons and in females. Recently another important factor has been identified - the HLA type of the individual patient plays a vital role. The HLA is a group of genes that encodes a group of proteins present on the surface of cells. HLA are involved in presenting foreign materials to the immune system. People who possess certain HLAs have a higher risk of ADRs with particular drugs (see Table 4). This is important to be aware of because the risk of an ADR with certain drugs can be reduced if you avoid giving the drug to patients with a high risk HLA.

Drug Factors Influencing Adverse Drug Reactions

Certain classes of drugs have a high incidence of adverse events. These include the nonsteroidal anti-inflammatory drugs (NSAIDS), antimicrobials (drugs helping fight bacteria), anticonvulsants (used to help control epilepsy), anaesthetic agents (An agent that causes loss of sensation with or without the loss of consciousness) and drugs that affect the muscles and nerves. You may be aware of recently published adverse events including the risk of cardiovascular disease (affecting the heart and blood vessels) with selective cox2 inhibitors, changes in the muscles with statins, angioedema (an allergic skin reaction characterized by patches of well defined swelling) with ACE inhibitors and angiotensin II antagonists, and lipid changes with antiretrovirals. There is convincing evidence that drugs associated with a high incidence of hypersensitive reactions are converted to products that can stimulate an immune mediated response in sensitive individuals. The way the drug is administered also influences the likelihood of an adverse event and its severity. Drugs applied to the skin are more likely to sensitize a patient to allergic reactions than drugs administered by injection or into the veins, and least often with the oral route. Intravenous administration gives rise to more severe reactions.

Diagnosis of an Adverse Drug Reaction

• Clinical HIstory

When evaluating the cause of an ADR, the doctor may ask some of the following questions. These include: any clinical symptoms experienced and their timing and duration in relation to drug exposure. The most serious clinical manifestation is anaphylaxis - a severe allergic response. This involves a drop in blood pressure, spasm of the airways in the lungs, swelling of the tissues within the skin and possibly collapse of the heart and blood vessels. These reactions occur within a short time of taking the drug (usually less than 3 hours) and are caused by Type I hypersensitivity (antibody mediated reactions). Drugs more commonly associated with these reactions are antibiotics, radiocontrast media, anaesthetic drugs, enzymes, cisplatin, insulin and latex. Dermatological skin changes are the most common side effect of many drugs. Table 5 summarises the common skin manifestations and their mechanisms. Other clinical manifestations are summarised in Table 6. Studies have shown the value of clinical history in differentiating drug allergy from other forms of adverse drug reactions. Two studies involving 638 and 319 patients who were referred to an allergy clinic for possible penicillin allergy were divided into three groups based on clinical history. In patients with a history suggestive of penicillin allergy, the likelihood of a positive skin test was 72.2% and 14.1%. In a second group with only a vague history of drug allergy the incidence of positive skin tests was 14.1% and 6.7%. In the third group with an unconvincing history of drug allergy only 0.9% and 0% were positive on skin testing. This area, however, is controversial with other studies providing conflicting data. Some experts therefore recommend that all patients with any history of penicillin allergy should be skin tested prior to receiving penicillin.

• Laboratory Investigations

Tests to determine possible ADRs which are mediated by the immune system include specific assays to measure the levels of antibodies present and also some skin tests. The diagnosis of drug allergy or hypersensitivity can be difficult and may require the input of a clinical specialist called an immunologist. The benefits of making an accurate diagnosis of an ADR include: 1.) Dangerous re- administration of drugs can be avoided 2.) Incorrect diagnosis are not made, allowing effective and inexpensive first line drugs to be administered. This avoids increased costs and the use of less efficacious second- line drugs and generation of antibiotic resistance. Testing in drug allergy clinics allows alternative drugs to be identified if required. If no alternative is available, a process called desensitisation can be performed - this involves gradual, repeated exposure to the allergic compound. Eventually, the immune system becomes tolerant of the compound and tolerance is maintained only for the course of therapy.

Treating an Adverse Drug Reaction

The most important decision for all suspected adverse drug reactions is to discontinue the suspected drug. In patients who are on multiple medications, the decision to stop the drugs must be weighed up between the need for the drug and its likelihood of causing the observed adverse effects. In suspected systemic Type I or anaphylactoid reactions the drug of choice to treat this ADR is adrenaline. It may be given as an injection into the muscle without delay, as early administration leads to better outcomes25. Another way of giving adrenaline is intravenously, through the veins, but this is only used in advanced, life threatening cases that fail to respond to adrenaline injections. Bronchodilators and systemic corticosteroids can also be used when treating allergic reactions, but administration of these should not delay the administration of adrenaline if required. Care should be taken with IV Phenergen (an antihistamine) because irritation of the veins can occur.