Medicines and side effects

Also called unwanted effects or adverse effects

Medicines are used for many health conditions. They can help people feel better, get well, prevent illness and even save lives. But all medicines, including prescription medicines, over-the-counter (OTC) medicines, herbal remedies, vitamins and supplements, can cause side effects.

Side effects, also called adverse reactions, are the unintended effects of a medicine. All medicines have possible side effects, but not everybody will experience them. When a medicine is recommended for you, it's important to ask questions about side effects and how they might apply to you.

It's important to report any side effects to your doctor, pharmacist or nurse. Reporting side effects helps protect everyone's health.

How common are side effects?

The likelihood of experiencing different side effects varies for each medicine. Some side effects are common and others are rare. Some can be managed or prevented and some are also more serious than others. You may not experience any side effects at all. 

The following terms are used to describe how likely a side effect is to occur, on average.

  • Very common (affects more than 1 person in 10).
  • Common (affects between 1 in 10 and 1 in 100 people).
  • Uncommon (affects between 1 in 100 and 1 in 1,000 people).
  • Rare (affects between 1 in 1,000 and 1 in 10,000 people).
  • Very rare (affects fewer than 1 in 10,000 people).
  • Not known (we don't know how many people are affected).

For example, if the risk is described as very rare (fewer than 1 in 10,000), that means 9,999 people out of 10,000 probably won't experience that side effect.

Note: Although this sort of information can give you an idea of how often side effects occur on average, it doesn't tell you how likely you personally are to experience a particular side effect. 

What is your risk of getting side effects?

Anyone can get side effects from a medicine, and it's difficult to know if a medicine will cause you side effects. Your risk of a side effect depends on several factors such as:

  • The dose of your medicine – you are generally more likely to get side effects when taking higher doses. 
  • How old you are – older adults are more likely to have side effects than younger adults.
  • Whether you are taking other medicines or herbal remedies that might interact with the new one.
  • Other health problems you may have. 

When do side effects occur?

You may notice side effects when you:

  • start taking a medicine
  • have been taking a medicine for a while
  • have a change in the dose of a medicine
  • stop taking the medicine.

If you notice a new symptom that starts within hours or days of taking a new medicine, it could be that the medicine is the cause. If you have a symptom months later it could be caused by something else, but some medicines do have delayed side effects that can happen later.

Will taking different medicines together cause side effects?

Taking some medicines together may cause side effects, or make them worse. This is called an interaction.

Interactions can happen with any medicines, including over-the-counter medicines, vitamin and mineral supplements, herbal or natural remedies and even food or alcohol. 

Before you take a new medicine or supplement, always ask your pharmacist about interactions with other medicines you are taking.    

How can side effects be prevented?

Some side effects can be avoided by following specific instructions for that medicine. For example, this may include:

  • starting with a low dose and increasing that dose gradually over time
  • taking the medicine with meals 
  • taking the medicine on an empty stomach 
  • taking the medicine at a particular time of the day
  • staying out of the sun.

The label on your medicine has important information including how much to take, how often to take the medicine and other special instructions. Learn more about medicine labels.  

Questions to ask about side effects

Knowing whether side effects can be managed may help you when deciding whether to start new medicines. When you're finding out about side effects, remember to weigh these up against the benefits of the medicine when you make any choices about treatment.

When finding out about side effects, important questions to ask your healthcare provider are:

  • What are the possible side effects?
  • How common are the side effects?
  • Are there any serious side effects and how likely are these?
  • Can I do anything to avoid or reduce the side effects?
  • Will the side effect get better with time?
  • What should I do if I am worried about a side effect?

Always discuss any side effects with your healthcare provider. If there are any side effects you're having trouble managing, there may be things your healthcare provider can suggest, such as reducing the dose of your medicine or trying another treatment.

Beware of unreliable information

Although you might be interested to hear about other people’s experiences with a medicine, remember that not everyone who takes the same medicine will have side effects, or have the same side effects. This is particularly important if you are searching online about your medicines. Remember that many online discussions about medicines and side effects are unreliable and may not apply to you. The best person to talk to is your healthcare provider.

What can I do if I experience a side effect?

If you are having trouble with side effects of a medicine, talk to your healthcare provider about it. There may be other ways to take the medicine or even other medicines that might suit you better.

Some side effects, eg, nausea (feeling sick), get better with time – some medicines make you nauseous for the first few days or even weeks, but the nausea then goes away. Other side effects don't go away, or don't occur straight away. 

Actions to take if you are experiencing a side effect
  • If the side effect is serious and it’s an emergency, call 111 for an ambulance.
  • If you're worried about the side effects of your medicine, talk to your doctor, nurse or pharmacist.
  • You can also call Healthline on 0800 611 116 for free advice from a registered nurse at any time – 24 hours a day, 7 days a week.
  • You can read about the side effects of your medicine in the consumer medication information leaflet on the Medsafe website. 
  • You can also check the warnings for natural and herbal products on the Medsafe website.

How can I report side effects?

If you think your medicine has caused a side effect, talk to your doctor. They may be able to lower your dose or change the medicine. Don't suddenly stop taking your medicine unless your doctor tells you to.

It's important to let your doctor, pharmacist or nurse know about your side effects. Reporting side effects helps protect everyone's health.

You can also report side effects yourself through the Centre for Adverse Reactions Monitoring (CARM). CARM accepts reports from consumers but will try to involve the doctor, nurse or pharmacist to make sure they are aware of the reaction.

CARM can be contacted through:

Learn more

Medicine and side effects NPS MedicineWise, Australia


  1. Adverse reactions to medicines Medsafe, NZ, 2019
  2. Medication side effects – how likely are they to occur? Sussex Partnership, NHS, UK 
  3. Understanding the frequency and severity of side effects: linguistic, numeric, and visual representations  American Association for Artificial Intelligence, US, 2006 
Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 10 Jan 2023