What Is the Difference between an Allergy and an Adverse Effect?


As a pharmacist, when you ask someone about their allergies, they will often tell you about medications that caused them to experience unpleasant, unexpected or intolerable effects. We of course want to know about these types of reactions, however, technically they are not allergies.

An allergy is an immunological reaction to an allergen. An allergen is a molecule that triggers an allergic reaction. Examples of allergens include pollen, peanuts, eggs, cat fur, sulphur, shellfish and some medications. Allergens are molecules or substances that are mostly harmless to the general population.

Your immune system is your body’s natural defence mechanism to fight off invaders, such as bacteria and other pathological bugs, to prevent infections. When your body identifies something as an allergen, it considers it an invader and reacts in a way to try to rid the body of it. The immune system then becomes hyperactive against this otherwise harmless ‘invader’.

This immunological reaction causes symptoms of an allergic reaction such as redness or hives, itching, swelling, shortness of breath, and nausea or vomiting. In extreme cases of anaphylaxis, your body can go into shock or you can experience swelling of the face, tongue or throat, which can block your airways.

Your immune system can become sensitive to the allergen, meaning subsequent exposure to the allergen can lead to more reactive and intense reactions. This is not always the case, as some people who developed mild allergies as a child to things such as antibiotics seem to grow out of the allergy and can tolerate them later as adults.

We can sometimes predict whether someone will have an allergic reaction to a medication if we know their medical history (ie, if they have eczema, hayfever or asthma that are usually triggered by allergens), their family history (ie, if anyone in their family has allergies) and whether they have had allergic-type reactions to anything in the past. Some medications have similar structures and if you are allergic to one you may also be allergic to another.

An adverse effect (also known as an adverse event or reaction) occurs when the desired effect of a medication or an inadvertent side effects causes a harmful, unpleasant or undesirable effect. For example, a blood-glucose-lowering medication used for diabetes may cause excessively low blood-sugar levels (known as hypoglycaemia), which could lead to a seizure or dizziness associated with a blood-pressure-lowering medication, which could then lead to a fall.

Adverse drug reactions can also be unpredictable or unexplained – eg, severe skin reactions or liver failure. These are often serious but rare. Unfortunately, it is unknown what triggers these reactions or why they occur and for this reason it is hard to predict who might experience one.

Some adverse effects can be more predictable and potentially avoidable. For example, some people may be more sensitive to the effects of certain medications due to their age or other medical conditions, or they might be taking other medications that have similar effects, making the adverse event more likely.

Side effects of medications are again slightly different to adverse effects. Medications are taken specifically to hit a target and cause a desired effect (also known as a therapeutic effect), while a side effect is another effect caused by the medication hitting ‘off’ targets. These side effects aren’t always detrimental or harmful and can in some cases be beneficial. For example, an antidepressant causing sleepiness can also be useful in someone with sleeping difficulties. A side effect can become an adverse effect when the outcome is unpleasant, harmful or detrimental. For example, a medication causing drowsiness in an elderly person could lead to them falling and hurting themselves.

So an allergy and an adverse effect are not the same but they are both just as important. Make sure your doctor and pharmacist know about all reactions you have had to medications or other substances, as this may influence how they treat you.

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