If you are having a medical test, your doctor may ask you to fast.
For some tests, fasting beforehand gives a more accurate result. For other tests, you need to fast for safety reasons.
Ask your doctor how to prepare for your test.
When you are fasting, you can't have anything to eat or drink.
Ask your doctor if you can drink plain water before your test.
Ask your doctor if you need to fast before a blood test. Some common tests that might require fasting are:
Usually, fasting blood test are done in the morning.
Avoid having anything else to eat or drink after your evening meal. For most people this means you will fast for 10 to 12 hours overnight.
A fasting blood test should be done after no less than 8 hours and no more than 16 hours of fasting.
You are usually allowed to drink water while fasting for a blood test. This means still plain water only. You can't drink tea, coffee, cordial, soft drinks or milk. Also, don't suck lollies or chew gum when fasting. Swallowing anything other than water can mean the blood test results will not be accurate.
Follow your doctor's instructions for how you should fast for your blood test.
If you are having sedation or a general anaesthetic for a test, you will need to fast beforehand. This lowers your risk of serious complications.
Your anaesthetist or surgeon will tell you when to stop eating and drinking.
Adults can usually keep having small amounts of clear fluids up to 2 hours before anaesthesia.
Clear fluids include:
You can't have jelly or milk while fasting.
There are different recommendations for babies and children. Ask your anaesthetist what your child can drink.
Fasting before a colonoscopy is important because:
Before the procedure, you will take a medicine to empty your bowel.
On the day before a colonoscopy, you will usually need to drink at least 3 litres of clear fluids. You won't be allowed to eat food.
You will need to fast for at least 2 hours before the procedure.
If you have diabetes, you may need to be monitored in hospital for this process.
Ask your doctor what to drink and if you should take your medicines. You should be given written instructions.
You must not eat or drink (including water) for 6 hours before a gastroscopy.
This gives your doctor a clear view to examine your stomach and intestines. Fasting also lowers your chance of complications from sedation.
Your doctor will tell you what time to start fasting.
Keep taking your medicines as usual before a test, unless your doctor tells you not to.
Some of your medicines may need to be stopped. Check with your doctor if you take regular medicines.
Tell your doctor about any over-the-counter and complementary medicines you take.
If you have diabetes, fasting can cause complications. You are more likely to have hypoglycaemia (low blood glucose levels or 'a hypo') when you are fasting.
Talk to your doctor before fasting. As for written instructions for you to follow while fasting. These should cover:
Here are some general tips for fasting in adults who have diabetes:
Call your doctor if you have any concerns or if you have to break your fast.
During your pregnancy you may be advised to have a fasting blood glucose test or a glucose tolerance test.
This test is to see if you have gestational diabetes (Opens in new tab). You will need to fast overnight for about 10 hours before these tests.
Read more about medical tests at Pathology Tests Explained (Opens in new tab).
Find information about how to manage diabetes while fasting at Diabetes Australia (Opens in new tab).
The Australian and New Zealand College of Anaesthetists (Opens in new tab) has information about preparing for procedures done under anaesthetic, such as a colonoscopy.
You can also call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available to speak with 24 hours a day, 7 days a week.
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Last reviewed: May 2025
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