Drugs prescribed for diseases are capable of causing harm to the consumer, even as they cure the original ailment. Almost all drugs may cause adverse reactions but many factors influence and alter the quality and severity of manifestation of these reactions.
The term ‘adverse reaction’ is generally used for effects that are harmful or cause much distress when used in recommended therapeutic doses. These effects usually disappear on discontinuing the drug. ‘Side effects’ is the term used for lesser reactions that occur when drugs are generally used in higher doses.
It is, at times, deemed quite difficult to determine whether a specific effect is actually caused by the drug, the disease process or whether it is an unrelated event. It is generally accepted that the causation is definite when the time period between the drug administration and occurrence of the reaction is reasonable; the effect is one that is known to be associated with the drug; stops on discontinuation and recurs on restarting the drug. The causation maybe deemed probable, possible, conditional or doubtful if the event fails to fulfill the above criteria.
There are different types of adverse reactions which maybe categorized as follows.
1. Type A reactions –
In the above, the patient experiences unwanted reactions which are exaggerated effects of the normal therapeutic actions of the drug. These occur when drugs are used in excessive amounts or when the concentration of the drugs in the body rises due to delayed excretion etc.
e.g. anti-diabetic drugs such as sulphonylureas may cause hypoglycaemic attacks
This type of reaction is predictable and related to drug dosage.
2. Type B reactions
These reactions occur only in some people and are either immune mediated or idiosyncratic. They are unpredictable and are not related to the dose.
e.g. allergy to penicillin and effects due to inherited diseases of the patient such as porphyrias
3. Type C reactions
These are relatively less common and are related to long-term use.
e.g. tardive dyskinaesia due to neuroleptic drugs
4. Type D reactions
Adverse reactions occurring after a period of time.
e.g. alkylating cytotoxics may lead to alterations in DNA and lead to the formation of cancers
5. Type E reactions
These occur following abrupt withdrawal of a drug
e.g. Adrenal crisis occurring following discontinuation of steroids
The adverse effects of drugs are discovered during clinical trials and post-marketing studies and surveillance systems are available to help detect previously unknown side effects. The clinician and patient must reach a compromise in which the benefits and risks of drug intake is discussed with a view to improving the quality of life of the patient.