Getting to know

The most frequently occurring adverse drug reactions are caused by giving too much of a drug (higher than the one recommended for the age, sex or condition of the person = overdose) or by giving other drugs at the same time (a very common situation today, especially in adults who suffer from multiple chronic diseases that need continuous treatment with different medicines). In these situations, adverse effects are in fact the toxic effects of the drug. These types of reaction are not allergic. 

True drug allergy is caused by an abnormal reaction of the immune system which results in production of antibodies capable of reacting to the drug. After these antibodies are produced, giving the drug again leads to its binding to the antibody and causing the release of chemicals that cause inflammation and allergic responses. (To learn more about the initiation and development of an allergic response please visit the section ‘What is allergy’) 

The immune system starts producing antibodies from the very first time it comes into contact with the drug (i.e. the first time you take the drug). However, allergic symptoms occur only when the immune system has produced enough antibodies to be able to trigger an allergic response. This usually takes several days (in more rare cases it can take weeks or months). 

Therefore, during the first days of treatment with the drug, the person will not have any symptoms (some people can even take several courses of treatment before showing any allergy symptoms). However, once a person has had an allergic attack, the symptoms occur immediately whenever the drug is taken again (severe reactions can occur within minutes of administration). 

Another important detail is that after the first allergic reaction has occurred, the severity of a new reaction is not related to the amount of drug given. Although there are greater chances of getting a severe reaction with a large amount of drug, sometimes even minute amounts of drug can cause severe allergic responses. 
To summarize:
Drug Allergic reactions

• Develop only if you have been exposed to the drug before. (Be careful:- sometimes you might have been in contact with the drug without knowing it. For example, you can be exposed to an antibiotic without knowing it by eating meat from an animal that has been treated with the antibiotic.)
• The first allergic episode can have a relatively slow onset, but the following ones will start immediately
• Reducing the dose of a drug does not prevent you from developing an allergic attack.  

Table 1: Differences between allergic and non-allergic adverse drugs reactions

Allergic drug adverse reactions	Non-allergic drug adverse reactions
Require previous exposure to the drug	Can occur with the first administration of the drug
First episode takes days to develop	Symptoms can develop from the first day of treatment
Reducing the dose does not prevent an allergic attack	Smaller doses usually prevent the adverse effect

Among adverse drug reactions there are some that mimic allergy very well but are not caused by antibodies reacting with the drug. These reactions are called pseudoallergies. 

So, if there is no allergy why are they the same? 
The reason is that drugs which cause pseudoallergies act on the cells of the immune system in a way that is not dependent on reacting with an antibody, but cause the release of the same substances as are released in a true allergic response.