CT With Contrast: What to Expect and How to Prepare

If your appointment letter says you are having a CT with contrast, a dye will be used during the scan to help parts of your body show up more clearly. The NHS calls this dye contrast medium.

This guide explains what CT contrast dye is, how it is given, what it feels like, how to prepare, the risks and what to do afterwards. Instructions vary between hospitals and clinics, so always follow the instructions your own clinic gives you.

What is CT contrast dye?

The NHS explains that contrast medium can be given in a drink, injected into a vein, or put in your bottom. University Hospitals Sussex says the injected dye contains iodine and helps staff see your internal organs and blood vessels. The Dudley Group says most people having its body CT scans have an injection of contrast dye to highlight their blood vessels and organs.

Not every CT uses contrast, and your clinic will tell you whether yours does. Our guides to the CT chest (thorax) scan and CT abdomen and pelvis explain those scans in more detail.

How the contrast is given

For an injection, a cannula (a small, thin plastic tube) is placed in a vein. The Dudley Group explains that the needle is then removed, leaving only the tube in place until the scan is finished.

For a CT abdomen and pelvis with contrast, you may also be given a drink. Royal United Hospitals Bath, for example, gives patients a contrast drink to take at set times on the two days before. The Dudley Group instead asks patients to drink some water 15 to 30 minutes before the scan. Your appointment letter will tell you what applies to you.

What a CT with contrast feels like

The NHS says that if contrast medium is injected you may feel hot and flushed, notice a metal taste in your mouth, or feel like you are peeing when you are not. These feelings usually pass quickly. The scan itself usually takes between 10 and 20 minutes, according to the NHS.

How to prepare: food, medicines and kidney function

The NHS says the hospital will tell you what to do, and that you may be asked to follow a special diet, take certain medicines, or not eat or drink for several hours beforehand. Hospitals differ:

• University Hospitals Sussex asks patients having a contrast injection not to eat for 2 hours before the scan.
• Royal United Hospitals Bath asks patients having a CT abdomen and pelvis not to eat for 2 hours before, but allows non-alcoholic drinks.
• The Dudley Group tells patients having its body CT scans that they can eat and drink normally beforehand.

Diabetes and metformin. University Hospitals Sussex says diabetes medicines such as metformin may need to be stopped before your scan. Royal United Hospitals Bath says it needs to know whether your kidney function is normal if you take metformin, and may ask you to stop it for 48 hours after the scan. Do not stop any medicine unless the clinic tells you to.

Kidney function. The Dudley Group asks patients with kidney problems who have not had a kidney function blood test in the last 2 to 3 months to contact the department first.

Other things to mention. The NHS asks you to say if you are pregnant or might be, are breastfeeding, have asthma, kidney or thyroid problems or diabetes, have any allergies, or have reacted to contrast medium before. See also how to prepare for your scan.

Contrast side effects and risks

The NHS says complications are uncommon, but some people have an allergic reaction to contrast medium, which can cause weakness, sweating and difficulty breathing. University Hospitals Sussex describes a reaction to the dye as very rare, and asks anyone with a known iodine allergy to contact it before their appointment.

The NHS says you may be asked to wait for up to 30 minutes after an injection so staff can check you do not react. The Christie NHS Foundation Trust lists symptoms to look out for in the 24 hours afterwards, including a rash, itching, dizziness, facial swelling, nausea, vomiting and difficulty breathing.

When to get urgent help: The Christie advises contacting your GP, NHS 111 or your nearest A&E if you get these symptoms after going home. The NHS says call 999 if you think someone is having an anaphylactic reaction, such as swelling of the throat and tongue or difficulty breathing.

Breastfeeding and aftercare

A 2022 joint statement from the Royal College of Radiologists and the Society of Radiographers says that stopping breastfeeding, or expressing and discarding milk, after iodinated contrast is not required. Tell the clinic you are breastfeeding so it can advise you.

After the scan, University Hospitals Sussex advises drinking plenty of clear fluids over the next 24 hours to help flush out the dye, and The Christie advises 2 to 3 litres of water over that time. If you were asked to stop metformin, The Christie asks you to arrange a kidney function blood test with your GP 2 to 4 days after the scan.

Radiation, referral and having a CT privately

CT uses X-rays. The UK Health Security Agency gives typical doses of about 8 mSv for a CT chest, roughly 3.6 years of natural background radiation, and about 10 mSv for a CT abdomen and pelvis, roughly 4.5 years. The NHS says that if you have lots of CT scans there is a very small chance the radiation could cause cancer. See our guide to X-ray and CT radiation safety.

Because CT uses ionising radiation, the Ionising Radiation (Medical Exposure) Regulations 2017 apply: every exposure needs a referrer and must be justified before it goes ahead. Many private clinics arrange this through their own doctor. Each scan is reported by an appropriately qualified clinician, and the clinic will tell you how long its report takes.

Prices vary by clinic, region, body area, whether contrast is used and what is included. FindScan shows each clinic's own listed price, so you can compare CT clinics near you and send an enquiry.