Side effects of medicines

Also called unwanted effects, adverse effects or adverse reactions

Medicines are a big part of the treatment for many health problems. 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 unwanted 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 require hospital admission.
  4. You are at increased risk of side effects if you are taking higher doses of a medicine, have other health problems and are on 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 suspected side effects to your doctor, pharmacist or nurse. Reporting side effects helps protect everyone's health and helps the safe use of medicines for babies, children and pregnant women.

How common are side effects?

Most side effects are minor, such as an upset tummy, but some can be serious, such as increasing your risk of a heart attack. The chance of getting side effects varies from person to person, and some side effects are more common than others.

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, approximately 5 of them may
get this side effect.
Uncommon If 1,000 people are given the medicine, approximately 5 of them may get this side effect.
Rare If 10,000 people are given the medicine, approximately 5 of them
may get this side effect.
Very rare If 100,000 people are given the medicine, less 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 there is no way 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 of a medicine
  • 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 after taking a medicine without problems for months, it could be caused by something else. But some medicines have delayed side effects that can happen after being on the medicine for quite a long time.

How severe are side effects?

Side effects can range from mild, such as drowsiness or feeling sick (nausea), to severe, life-threatening conditions that require hospital admission, such as a severe allergic reaction (called anaphylaxis). This is a potentially life-threatening allergic reaction that usually occurs within 30 minutes of taking the medicine, but can occur up to several hours later. Common symptoms are difficulty breathing or talking, swelling of your tongue and tightness in your throat. Read more about severe allergic reactions (anaphylaxis) to medicines.   

Some side effects go away after a while, once your body gets used to the medicine, but some side effects can last much longer and be troublesome. 
Side effects may be different for different people, eg, some cold and flu medication can cause drowsiness, which may be helpful if you want bed rest, but may be unwanted if you need to be alert, such as for driving or operating machinery. Also, although dizziness may not seem like a serious side effect, it can be very difficult for older adults or those already unsteady on their feet, because it can increase the risk of falls.

Will taking 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 prescription medicines, over-the-counter medicines, vitamin and mineral supplements, herbal or natural remedies, food and alcohol or illegal drugs. Before you take a new medicine or supplement, always ask your pharmacist about any interactions.  

How can side effects be prevented?

There are many things you can do to prepare for and prevent 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 printout of information about your medicines.

Prevent side effects

  • Let your doctor, nurse or pharmacist know if you have any allergies to medicines.
  • Find out if you can do anything to prevent side effects, 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 how well your kidneys are working.
  • Check if you should avoid some supplements, foods or alcohol while taking your medicine.

Where can I get information on side effects?

If the side effect is serious and it’s an emergency, such as affecting your breathing, 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 to get advice from a registered nurse at any time – 24 hours a day, 7 days a week.
  • You can read about the side effects of a particular medicine in the consumer medication information leaflet on the Medsafe website
  • You can also check for warnings on natural and herbal products on the Medsafe website.

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

While medicines are safe and effective, side effects can happen. All medicines carry some risk. The risks may be minor and the benefit of the medicine to your health may outweigh the risk. But the benefit–risk decision is sometimes difficult to make. The best choice depends on your situation. You must decide what risks you will accept to get the benefits you want.

For example, if you have a life-threatening illness, you might choose to accept more risk in the hope of a cure or living a longer life. But if you have a minor illness, you might decide that you want to take very little risk. Your doctor, pharmacist or other healthcare professional 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 switch you to another medicine. Don't suddenly stop taking your medicine unless your doctor tells you to.

It is important to report suspected side effects to your doctor, pharmacist or nurse. Reporting side effects helps protect everyone's health and helps the safe use of medicines for babies, children and pregnant women.

You can also report side effects through an organisation called Centre for Adverse Reactions Monitoring (CARM). CARM accepts reports from consumers but will try to involve the patient's doctor, nurse or pharmacist who often may be unaware of the reaction. CARM can be contacted through:

Learn more

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

References

  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 
Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 24 Oct 2017