Adverse Drug Reactions: An Overview

Approximately 400,000 Australians are hospitalised each year for medicine-related reasons. Between 7-11% of these hospital admissions are caused by adverse drug reactions (ADRs), more than half of which are considered preventable (Tagna et al. 2023).

Safe medicine use is central to preventing patient harm. Therefore, any known or new adverse drug reactions should always be documented, monitored and communicated appropriately to prevent re-exposure (SA Health 2024).

Although some ADRs are unpredictable, many can be avoided (SA Health 2024).

What Are Adverse Drug Reactions (ADRs)?

Adverse drug reactions (ADRs) are unwanted or harmful reactions to medicines that were taken under normal conditions of use. These reactions are related to the medicines themselves rather than other factors (Cosgrave et al. 2025).

All medicines, including complementary and natural medicines, can cause ADRs (ASCIA 2023).

ADRs can range from minor, uncomfortable side effects to dangerous and potentially life-threatening reactions (Kommu, Carter & Whitfield 2024). It's important to distinguish a true allergy from an intolerance or an expected side effect. Allergies are immune-based reactions such as hives, swelling, breathing difficulty or anaphylaxis. Intolerances are non-immune reactions like nausea or headache, and side effects are the predictable effects of the medicine's action. Mixing these up can lead to unnecessary medicine avoidance or missed safety risks, so clear documentation is essential (SA Health 2024).

While an ADR refers specifically to harm caused by a drug's pharmacological action at normal doses, the broader term adverse drug event (ADE) encompasses any harm associated with a medicine, including errors in prescribing, dispensing, distribution, or administration (SA Health 2024). In other words, an ADE is any unwanted occurrence that follows drug exposure, regardless of the cause (Cosgrave et al. 2025).

Distinguishing the two matters for clinical management, safety monitoring, and reporting:

• ADR: The drug works as expected, but causes harm at normal doses (e.g. gastritis from NSAIDs)
• ADE: Harm arises from any unintended consequence of drug use, including errors or overdose (e.g. liver failure from a paracetamol overdose).

(Cosgrave et al. 2025)

Adverse Drug Reactions in the National Safety and Quality Health Service Standards

ADRs are outlined in Actions 4.07, 4.08, and 4.09 of the National Safety and Quality Health Service Standards, under Standard 4: Medication Safety.

These actions aim to minimise the risk of ADRs occurring by directing all healthcare providers to establish processes for:

• Documenting patients' medication and ADR histories
• Recording any ADRs that occur during an episode of care in the healthcare record, as well as in an organised, wide incident reporting system
• Reporting these events to the TGA.

(ACSQHC 2021)

Adverse Drug Reactions Under the Strengthened Aged Care Quality Standards

Standard 5: Clinical Care - Outcome 5.3: Safe and quality use of medicines under the Strengthened Aged Care Quality Standards(Action 5.3.5) advises registered aged care providers to establish processes for reporting adverse effects of medicines and vaccines to the Therapeutic Goods Administration (TGA) (DoHDA 2025).

Types of Adverse Drug Reactions

Most ADRs can be divided into two main categories:

Type A ADRs are predictable, dose-dependent reactions related to the pharmacology of the particular medicine (e.g. constipation caused by a medicine that prevents diarrhoea). They are the most common type of ADR. Type A reactions include:

• Toxic effects
• Side effects
• Secondary effects
• Medicine interactions.

Type B ADRs are hypersensitivity reactions (e.g. allergic reactions). They are unpredictable and not dose-dependent. Type B reactions account for about 10 to 15% of ADRs.

(SA Health 2024; Kommu, Carter & Whitfield 2024)

Risk Factors for Adverse Drug Reactions

Based on the timing of their onset, allergic reactions are categorised as 'immediate', appearing within one to six hours of administration, or 'non-immediate', developing at least 24 hours after administration (ASCIA 2023).

The following factors may increase the risk of experiencing an ADR:

• Age (children and older adults are more at risk due to differences in the renal, hepatic and cardiovascular systems)
• Polypharmacy
• Sex (some medicines are more likely to cause anaphylaxis in females)
• Smoking, which increases the risk of anaphylaxis
• Atopy, which is linked to latex allergy and anaphylaxis
• Exposure to latex.

(Patton & Borshoff 2018)

Certain medicine classes contribute disproportionately to serious and preventable ADRs, particularly anticoagulants, opioids, insulin, antipsychotics and cardiovascular medicines. Errors with these medicines are not necessarily more frequent than with others, but because they have a narrow margin of safety, the consequences can be far more severe (ACSQHC 2021).

Symptoms of Adverse Drug Reactions

Based on the timing of their onset, allergic reactions are categorised as 'immediate', appearing within one to six hours of administration, or 'non-immediate', developing at least 24 hours after administration (ASCIA 2023). The severity and risk of ADRs vary depending on the medicine and the individual (Medsafe 2023).

Examples of common ADRs may include:

• Nausea
• Upset stomach
• Skin itchiness, rash, hives and welts
• Drowsiness
• Headache
• Sore injection site
• Asthma and hay fever
• Xerostomia (dry mouth).

Examples of severe ADRs may include:

• Anaphylaxis
• Severe, non-immediate rashes
• Severe asthma
• Fever, flu-like symptoms.

(Healthdirect 2024; ASCIA 2023)

Please note that the above lists are not all-inclusive, and patients should be encouraged to report all adverse reactions, regardless of severity.

Information about common and significant ADRs for a specific medicine can be found in that medicine's consumer medicines information (CMI) leaflet. There is a CMI for every prescription medicine and some non-prescription medicines. As well as listing potential ADRs, CMI leaflets contain other important information about the medicine, including dosages, interactions, and storage conditions (TGA 2020).

CMIs are listed on the Australian Register of Therapeutic Goods.

Diagnosing Adverse Drug Reactions

If symptoms develop soon after a medicine has been administered, an ADR may be easy to recognise. However, ADRs caused by chronic medicine use are more difficult to diagnose. Furthermore, ADRs can present with a wide variety of symptoms, some of which may be misattributed to other illnesses. Identifying the cause often requires a thorough medication history, a thorough assessment and review, and, where safe, a monitored withdrawal trial (S. Lynch 2025; Kommu, Carter & Whitfield 2024).

Documenting Adverse Drug Reactions

Any known ADRs should be documented during a patient's medical history. New ADRs should be documented when they occur, in accordance with your organisation's policy (SA Health 2024).

ADR records should be checked before any medicine is prescribed, dispensed or administered to the patient. Ideally, the documentation should include:

• Generic and brand name of the medicine
• Type of ADR experienced by the patient
• Date of the ADR reaction or approximate timeframe
• How the ADR was managed
• Length of time between exposure to the medicine and the onset of the reaction
• Class effect ADR
• Source of this information, and whether the ADR has been confirmed by a health professional
• Signature or initials of the individual documenting.

(SA Health 2024)

If the patient has no known history of ADR, this should be documented (SA Health 2024).

Preventing Adverse Drug Reactions

The best way to prevent ADRs is to identify patients who are likely to be susceptible. Their treatment plan should aim to mitigate potential adverse effects (Kommu, Carter & Whitfield 2024).

Other steps involved in preventing ADRs include:

• Obtaining a thorough medication history by engaging not only the patient but also family members and carers, who may provide valuable information
• Identifying any potential drug interactions when reviewing the patient's medication
• Promoting medication adherence by using the teach-back method and open-ended questioning to confirm the patient understands how and when to take their medications
• Providing written materials such as handouts or pamphlets, ideally in the patient's preferred language
• Educating patients on how to recognise the signs and symptoms of adverse drug reactions.

(Kommu, Carter & Whitfield 2024; Roy & Rogers 2025)

To prevent a known ADR from recurring, the patient's treating team may decide to discontinue the medicine or replace it with an alternative.

Reporting Adverse Drug Reactions

ADRs should be reported to the Therapeutic Goods Administration (TGA). The TGA is responsible for operating a safety monitoring program that relies on information submitted by both the general public and health professionals (TGA 2025).

• Any suspected ADRs caused by a new therapeutic good
• Any suspected medicine and/or vaccine interactions
• Any unexpected ADRs that do not appear in the medicine's CMI, Product Information or product labelling
• Any serious ADRs, including those suspected of causing:
  • Death
  • Life-threatening injury or illness
  • Hospitalisation
  • Prolonged hospitalisation
  • Absence from work or daily activities
  • Increased investigational or treatment costs
  • Birth defects.

(TGA 2025)

For more information on reporting an adverse event to the TGA, see: Overview of adverse event reporting for medicines.

Conclusion

Although some ADRs are unavoidable, many are preventable with the right safeguards in place. Thorough documentation, vigilant monitoring and clear communication at every stage of prescribing, dispensing and administering medicines are essential to reducing patient harm. Reporting suspected ADRs to the TGA further supports the ongoing safety of medicines for all Australians (TGA 2025).