Quick answer: For most X-rays, CT, MRI and ultrasound scans, you can take your usual medicines as normal. But some medicines matter for specific tests: diabetes medicines before PET scans, fasting or contrast; heart medicines and caffeine before nuclear stress tests; thyroid medicines before thyroid scans; blood thinners before biopsies or procedures; medicines given before sedation; and medication patches before MRI. Never stop or change a medicine on your own. Bring a full list of your medicines and ask the questions below.
The golden rule
Some medicines are dangerous to stop suddenly, such as heart, blood pressure, seizure, blood-thinning and steroid medicines. Only your doctor or the imaging team, who know your health and the test, should tell you whether to pause or change anything. If you get different instructions from different people, call the imaging center and ask who has the final say.
Medicines that commonly come up
Diabetes medicines (insulin, metformin and others)
- Why it matters: Many scans ask you to fast, which affects blood sugar. PET scans need stable, not-too-high blood sugar to work well. Some diabetes medicines may be adjusted if you get CT contrast and have low kidney function.
- Ask: «I take [medicine] for diabetes. How should I handle it before this scan, given that I need to fast?»
Weight-loss and diabetes injections (GLP-1 medicines)
- Why it matters: Medicines such as semaglutide slow stomach emptying. If your scan or procedure involves sedation or anesthesia, food left in the stomach can be a safety risk. Medical guidance on this is still evolving.
- Ask: «I take a GLP-1 medicine. Does that change anything for my sedation or fasting?»
Blood thinners and antiplatelet medicines
- Why it matters: For most scans, they don’t matter. For biopsies, joint injections, lumbar punctures and other procedures, bleeding risk does. The decision balances bleeding against clotting risk and should involve the doctor who prescribes your blood thinner.
- Ask: «Is my procedure one where my blood thinner matters? Who will decide, and will my prescribing doctor be involved?»
Heart medicines and caffeine before a nuclear stress test
- Why it matters: Some heart medicines and caffeine can interfere with how the stress test works, depending on the type of test.
- Ask: «Do any of my heart medicines or my coffee, tea or energy drinks affect this test?»
Thyroid medicines before a thyroid scan or radioactive iodine
- Why it matters: Thyroid medicines, iodine supplements and recent iodine contrast can change how the thyroid takes up the tracer.
- Ask: «Do my thyroid medicines or supplements affect this scan? Have I had iodine contrast recently that could matter?»
Medication patches before MRI
- Why it matters: Some skin patches (for pain, nicotine, hormones, motion sickness and others) contain metal that can heat up in the MRI scanner and cause burns. The FDA has warned about this.
- Ask: «I wear a [type] patch. Should it come off for the MRI, and how do I replace it afterward?»
Insulin pumps, glucose sensors and other devices
- Why it matters: Many devices aren’t safe in the MRI room or need to be removed for CT or X-ray in the scanned area.
- Ask: «How should I manage my pump or sensor during and after the scan?»
Medicines for anxiety, pain or sedation
- Why it matters: If you’re given or prescribed something to relax for the scan, you usually can’t drive afterward.
- Ask: «Will I need someone to drive me home?»
Steroids, antihistamines and premedication for contrast allergy
- Why it matters: If you’ve had a reaction to contrast before, your doctor may prescribe medicine to take beforehand with specific timing.
- Ask: «Exactly when should I take my premedication, and what if I miss a dose?»
Supplements
- Why it matters: Some supplements, such as those containing iodine or kelp, matter for thyroid tests. Most others don’t affect imaging, but your team should know what you take.
Your pre-scan checklist
- A full, up-to-date list of medicines, including doses, patches, inhalers, injections and supplements
- Clear instructions about any medicine to change, from your doctor or the imaging center
- Instructions for diabetes medicines if you need to fast
- A plan for patches, pumps or sensors if you’re having an MRI
- Premedication instructions, if you’ve had a contrast reaction
- A ride home if you’ll get sedation
From the scanner room
[TU EXPERIENCIA: los problemas de medicación que más ves en tu sala (parches en RM, cafeína antes de una prueba de esfuerzo nuclear, glucosa alta antes de un PET, tratamiento tiroideo antes de una gammagrafía) y cómo los resolvéis, sin datos personales.]
FAQ
Can I take my medicines before an MRI?
Usually yes, with water. If you wear a medication patch, ask whether it needs to come off.
Can I take my medicines before a CT scan with contrast?
Usually yes. If you take diabetes medicine and have kidney problems, your team may give you specific instructions.
Should I take insulin before a PET scan?
Ask your PET center. Diabetes medicines are often adjusted for PET scans, and the center will give you a plan based on your medicines and blood sugar.
Do I need to stop blood thinners before a scan?
Not for most scans. For biopsies and some procedures, your doctors may decide to adjust them. Never stop them on your own.
What if I took my medicine by mistake?
Call the imaging center before your appointment. In many cases the scan can still go ahead or be rescheduled safely.
This article is for general information only and is not medical advice. Always follow the instructions from your doctor and imaging center, and don’t stop or change any medicine without their guidance.
Sources
- FDA. Public health advisory: risk of burns during MRI scans from transdermal drug patches with metallic backings (2009).
- American Society of Anesthesiologists and multisociety guidance on GLP-1 receptor agonists before procedures (2023–2024).
- ACR Manual on Contrast Media.
- SNMMI procedure standards (PET, thyroid and myocardial perfusion imaging).
- RadiologyInfo.org (ACR/RSNA). Patient preparation pages.