Medicines and side effects

Also called unwanted effects or adverse effects

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

Key points

  1. All medicines can cause side effects. Some side effects are more common than others.
  2. The chance of getting side effects varies from person to person. 
  3. Side effects can range from mild, such as drowsiness or feeling sick (nausea), to severe, life-threatening conditions that need to be treated in hospital.
  4. You are at increased risk of side effects if you are taking higher doses of a medicine, have other health problems or are taking other medicines.
  5. While you may not be able to prevent all side effects, knowing what to expect and how to deal with them can help.

It is 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?

Most side effects are minor, such as an upset tummy, but some can be serious. The chance of getting side effects varies from person to person, and some side effects are more common than others. Side effects are often described in the following ways:

Description Scale used to describe side effects1
Very common If 10 people are given the medicine, at least 1 of them is likely to get this side effect.
Common If 100 people are given the medicine, about 5 of them may
get this side effect.
Uncommon If 1,000 people are given the medicine, about 5 of them may get this side effect.
Rare If 10,000 people are given the medicine, about 5 of them may get this side effect.
Very rare If 100,000 people are given the medicine, fewer than 10 of them are likely to get this side effect.

What is my risk of getting side effects?

Anyone can get side effects from a medicine, and it is difficult to know if a medicine will cause you side effects. It may depend on:

  • the dose of your medicine – you are 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
  • 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.

How bad are side effects?

Side effects range from mild, such as drowsiness or feeling sick (nausea), to severe, life-threatening conditions that need to be treated in hospitalSome side effects go away once your body gets used to the medicine, but others can last much longer. 

Some people might not mind certain side effects, but for others they can be difficult to manage, eg, some cold and flu medicines can cause drowsiness, which may be helpful if you want to rest, but unwanted if you need to be alert for work and driving. Dizziness can be dangerous especially for older adults because it can increase the risk of falls and fractures.

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, alcohol or illegal drugs. 

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?

There are many things you can do to manage side effects. Before you take any medicine, talk to your doctor or pharmacist.

Prepare – ask about possible side effects

  • What are the common side effects to expect?
  • How should you deal with any side effects?
  • How soon may the side effects start?
  • Will the side effects go away on their own?
  • Will you need any tests to check for side effects?
  • What can you do to manage mild side effects?
  • When and who should you call for help with more serious side effects?

You can ask your pharmacist for a printed information about your medicines.

Prevent side effects

  • Let your doctor, nurse or pharmacist know if you have had side effects before.
  • Find out if you can do anything to prevent side effects, or make them easier to manage, such as taking a medicine with food or at a certain time of day.
  • Ask if you need any tests to check if you are at risk of side effects, such as checking your kidneys.
  • Check if you should avoid some supplements, foods or alcohol while taking your medicine.

Where can I get information about side effects?

If the side effect is serious and it’s an emergency, call 111 for an ambulance.
  • If you are 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 for warnings of natural and herbal products on the Medsafe website.

How do I weigh up the benefits and risks of medicines?

While medicines are usually safe and effective, side effects can happen. All medicines have some risk. The risks may be minor and the benefit of the medicine to your health may be worth the risk. But the benefit–risk decision is sometimes difficult to make and depends on your situation. Your doctor, pharmacist or other healthcare provider may be able to help you make the decision.

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 is 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

Medicines and falls risk Health Navigator, NZ
Medicines and sexual problems Health Navigator, NZ
Medicines and weight gain Health Navigator, NZ
Severe allergic reactions (anaphylaxis) to medicines (Maori), Medsafe, NZ 
Medicines for depression or other mental disorders and difficulties with sex (sexual dysfunction) Medsafe, NZ


  1. Medication side effects – how likely are they to occur? Sussex Partnership, NHS, UK 
  2. What are side effects? NHS, UK
  3. Understanding the frequency and severity of side effects: linguistic, numeric, and visual representations  American Association for Artificial Intelligence, US, 2006 

Useful resources for healthcare professionals

Adverse Drug Reactions Medsafe, NZ

Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 18 May 2020