Revised: 21 November 2023

Safety Information

Adverse Reactions to Medicines and Vaccines

What is an adverse reaction?
What are adverse events following immunisation?
What is meant by a common or rare adverse reaction?
Reducing the risk of adverse reactions
When do adverse reactions occur?
Reporting adverse reactions
More information

What is an adverse reaction?

Adverse reactions are harmful effects suspected to be caused by a medicine or vaccine. They are also known as adverse drug reactions (ADRs), side effects or undesirable effects.

All medicines and vaccines can cause adverse reactions in some people. Adverse reactions can range from headaches and upset stomach to more serious reactions such as liver or kidney injury. Some adverse reactions can be predicted, but some adverse reactions occur unexpectedly once many people take the medicine (eg, severe allergy). Most people take medicines without suffering any serious adverse reactions.

The best way to know what adverse reactions may be experienced is to read the medicine data sheet and/or consumer medicine information available on the Medsafe website.

Find a medicine data sheet or consumer medicine information

What are adverse events following immunisation?

Adverse reactions to vaccines are sometimes referred to as adverse events following immunisation (AEFI). The following types of adverse reactions are expected to be reported with any vaccine.

  • Injection site reactions (eg, pain, swelling).
  • 'Well-recognised' events such as headaches, dizziness, muscle aches, mild fever and tiredness.
  • Mild allergic reactions such as mild rashes and itching.
  • A few serious allergic reactions (healthcare professionals giving vaccines are trained to spot the symptoms of serious allergic reactions and treat them).
  • Events due to fear or anticipation of the needle injection, such as fainting.
  • Coincidental medical conditions (eg, neurological disorders).
  • New adverse reactions (ie, those not already listed in the data sheet).

In New Zealand it is difficult to detect rare side effects to vaccines as the population immunised is usually too small. Therefore, Medsafe uses international data to help determine if these rare events may be due to immunisation.

What is meant by a common or rare adverse reaction?

The chance of having an adverse reaction can be described as:

  • very common – this means that 1 in every 10 people taking the medicine are likely to have the adverse reaction
  • common – this means that between 1 in 10 and 1 in 100 people may be affected
  • uncommon – this means that between 1 in 100 and 1 in 1,000 people may be affected
  • rare – means that between 1 in 1,000 and 1 in 10,000 people may be affected
  • very rare – means that fewer than 1 in 10,000 people may be affected.

If an adverse reaction affects 1 person in every 10,000 people taking it, then 9,999 people out of 10,000 are not expected to have that adverse reaction.

Reducing the risk of adverse reactions

  • Always take medicines as advised by a pharmacist, doctor or prescriber (eg, some medicines that can cause drowsiness are best taken at night).
  • For over-the-counter medicines (eg, purchased without healthcare professional advice), always follow the directions on the package.
  • Never take medicines prescribed for other people.
  • Never take more medicine than recommended.
  • Be careful about mixing medicines because some medicines should not be taken together. Tell your doctor or pharmacist about all the medicines you are taking (including prescription medicines, medicines bought in a pharmacy or supermarket, complementary medicines and food supplements) so they can advise you accordingly.
  • Be careful about taking medicines with alcohol or certain foods.

When do adverse reactions occur?

There are many factors affecting when adverse reactions may occur.

In general, adverse reactions are most likely to happen soon after medicines are started or after increasing the dose. Other adverse reactions can occur after long term use. A patient's doctor can monitor for these reactions to prevent them occurring (eg, blood tests with warfarin). Most reactions to vaccines occur within a very short time after vaccination, usually within days.

Some adverse reactions will go away if the medicine is continued. However, any concerns should be discussed with a doctor or pharmacist. You should not make any changes to your medicine or stop taking your medicine without first talking to your doctor or a pharmacist.

Reporting adverse reactions

Anyone living in New Zealand who thinks they may have experienced an adverse reaction due to a medicine or vaccine can report it. You do not need to be a healthcare professional to report an adverse reaction.

Medsafe collects and processes suspected adverse reaction reports and the physicians at the Centre for Adverse Reactions Monitoring (CARM) medically assess non-routine reports.

Report an adverse reaction to a medicine or vaccine

More information

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