What medications should I avoid for a PET scan?

October 6, 2025 •

5 min read

According to the Journal of Nuclear Medicine, proper patient preparation is a critical step towards obtaining good quality images and accurate results during a PET scan. This guide explains what medications should I avoid for a PET scan and offers a comprehensive overview of how certain drugs, particularly diabetes medications, can interfere with the imaging process.

Quick Summary

A PET scan requires careful medication and diet management, particularly for those with diabetes. Insulin and metformin must be temporarily discontinued to prevent interference with the radioactive tracer. Other medications may also need adjustment, and consulting your doctor is essential for accurate test results.

Key Points

Diabetes Medications are Critical: Insulin and metformin must be managed carefully before an FDG-PET scan, as they directly alter the body's glucose metabolism and can compromise image quality.
Coordinate with Your Physician: Always consult your prescribing doctor or the imaging center for specific instructions regarding medication adjustments; never stop taking a prescribed drug abruptly on your own.
Timing is Essential: The duration for which you must hold a medication, particularly insulin or metformin, depends on the type of drug, your scan time, and the purpose of the exam.
Consider Other Influencing Drugs: Beyond diabetes medications, corticosteroids, some heart medications, and substances like nicotine can also affect the scan and may require temporary cessation.
Timing After Treatment: Chemotherapy and radiation can cause temporary metabolic changes. The timing of a post-treatment PET scan is often delayed to ensure the most accurate results.
Bring Your Medication List: On the day of your scan, provide the technician with a complete list of all medications, including over-the-counter drugs and supplements, for the most accurate review.

A Positron Emission Tomography (PET) scan is a powerful diagnostic tool that uses a radioactive tracer, often a form of glucose called fluorodeoxyglucose (FDG), to visualize the metabolic activity within the body. Because this tracer mimics glucose, any factor that affects your body's glucose and insulin levels can compromise the scan's accuracy. This is why certain medications, primarily those related to diabetes management, must be temporarily avoided or adjusted before the procedure.

Diabetes medications to withhold before a PET scan

Diabetes medications are the most common cause of interference with a PET scan, as they directly impact your body’s glucose metabolism. High blood sugar levels can compete with the FDG tracer, reducing its uptake by cancer cells or other areas of interest and leading to poor image quality or inaccurate results. Conversely, altering insulin levels can also change tracer distribution throughout the body. Specific instructions will vary, but some common precautions are outlined below.

Metformin

Metformin, a common oral medication for Type 2 diabetes, is known to significantly increase FDG uptake in the bowel. This can obscure potential abnormalities in the abdomen and pelvis, leading to misinterpretations of the scan.

General recommendation: For most FDG-PET scans, temporary cessation of metformin prior to the exam may be recommended.
Patient communication: Patients should always discuss the exact timing with their prescribing physician or the PET facility, as the protocol can depend on the specific scan and clinical situation.

Insulin and other oral diabetic agents

Management of insulin and other oral agents (e.g., sulfonylureas, GLP-1 agonists) is highly specific and depends on the type of medication and your scan schedule.

General recommendations: For many patients, the morning dose of insulin or other oral diabetic medications may need to be skipped on the day of the scan. Long-acting insulin or insulin pump settings may require adjustments in the days leading up to the appointment.
For morning scans: Patients often fast after midnight and hold all diabetes medications until after the exam is complete.
For afternoon scans: Patients might have a light, high-protein, low-carbohydrate breakfast at least 6 hours before the scan, taking their oral diabetic medication at that time. Insulin users should follow specific instructions.

Other medications and supplements that can interfere

Beyond diabetes medications, other substances can also affect the results of an FDG-PET scan by influencing glucose metabolism or tracer uptake.

Steroids (Corticosteroids)

Corticosteroids like prednisone can cause high blood sugar levels (hyperglycemia), which interferes with FDG uptake. This can suppress the inflammatory response or metabolic activity being studied, potentially leading to false-negative results.

Action required: Discontinuation is generally recommended prior to the scan, depending on the dose and the clinical reason for the scan. Patients should only stop these medications under a doctor's guidance due to the risk of side effects.

Certain heart medications

For cardiac PET scans, some heart medications may need temporary adjustment. For example, nitrates and beta-blockers can affect blood flow to the heart and alter tracer distribution.

Action required: For a cardiac stress test, beta-blockers, nitrates, and erectile dysfunction drugs may need to be held beforehand to allow the heart rate to respond adequately during the stress portion of the exam.

Sympathomimetics

Drugs that stimulate the sympathetic nervous system, such as ephedrine (found in some over-the-counter decongestants) and nicotine, can increase metabolic activity in brown adipose tissue (brown fat). This can create distracting hot spots on the scan and interfere with interpretation.

Action required: It is best to avoid these substances for a period prior to the exam.

Chemotherapy and colony-stimulating factors

Chemotherapy and drugs like Neulasta that stimulate bone marrow can cause inflammatory changes and alter metabolic activity. This can cause a high FDG uptake in the bone marrow, making it difficult to detect other issues.

Action required: The timing of the PET scan after a cycle of chemotherapy or after receiving a colony-stimulating factor injection is crucial. Scans are often scheduled to avoid false readings.

Comparison table: Medications and their impact on FDG-PET scans


Medication Class	Why It Interferes	How to Manage
Metformin	Increases FDG uptake in the bowel, which can hide lesions in the abdomen.	Temporary cessation before the scan. Always consult your doctor.
Insulin	Directly affects blood sugar levels, competing with the FDG tracer and altering its biodistribution.	Adjust dosage or hold on the day of the scan, based on appointment time. Follow specific diabetic prep instructions carefully.
Oral Hypoglycemics (other than Metformin)	Can significantly alter blood glucose levels.	Hold on the morning of the exam, or follow modified instructions provided by the imaging center.
Corticosteroids	Raise blood sugar levels, leading to poor tracer uptake and potentially false results.	Taper off or discontinue prior, only with a doctor's permission.
Beta-Blockers and Nitrates (for Cardiac PET)	Can alter blood flow to the heart during a stress test, affecting diagnostic accuracy.	Hold before a cardiac stress test, as advised by your physician.
Sympathomimetics (e.g., Ephedrine, Nicotine)	Stimulate brown fat, creating distracting tracer uptake areas on the scan.	Avoid for a period before the exam.
Chemotherapy	Induces inflammation and changes in metabolic activity, potentially causing false-positive or false-negative results.	Time the scan after the last treatment session, as advised by your oncologist.

The importance of communication with your doctor

While this article provides general guidelines, each patient's situation is unique. The single most important step in preparing for a PET scan is to communicate openly with your physician and the imaging center staff. They will provide specific, personalized instructions based on your medical history, the type of PET scan being performed, and the specific medications you are taking.

Provide a complete medication list: Bring a comprehensive list of all prescription medications, over-the-counter drugs, vitamins, and supplements.
Don't stop medications abruptly: Never stop or change the dose of any medication without first consulting your doctor, as this could be unsafe.
Timing is everything: Discuss the timing of your last dose for any potentially interfering medications to ensure the scan can proceed safely and accurately.

Conclusion

To ensure the highest accuracy of your PET scan, avoiding certain medications is a necessary step, especially those that affect blood glucose levels. Medications for diabetes, such as insulin and metformin, are key examples that require careful management in consultation with your doctor. Other drugs, including corticosteroids and some cardiac medications, may also need to be adjusted or withheld based on the type of scan. The guiding principle is to follow your medical team's instructions precisely and to provide them with a full list of all medications you take. By doing so, you will help guarantee that the images produced are clear and diagnostically reliable..

Journal of Nuclear Medicine Technology

Frequently Asked Questions

A PET scan uses a glucose-based radioactive tracer (FDG) to highlight areas of high metabolic activity in the body, such as tumors. Medications that alter blood sugar levels, such as insulin or metformin, can interfere with how the body absorbs this tracer, leading to inaccurate or unclear images.

In most cases, you can continue taking regular, non-diabetic prescription medications with water. However, you should always provide a complete list of all medications, vitamins, and supplements to the imaging center staff for their review.

For an FDG-PET scan, you will typically be advised to stop taking metformin before the test. This is to prevent increased tracer uptake in the bowel, which could interfere with image interpretation.

Managing insulin before a PET scan is highly specific. Depending on your appointment time, you may need to skip your morning dose. If you use a pump, it may need to be adjusted. Always consult your physician for a personalized plan to ensure safe blood sugar management.

Yes, avoid over-the-counter medications that contain caffeine or decongestants, such as ephedrine, for a period before the scan. Nicotine products should also be avoided as they can affect tracer uptake.

Yes, certain sedatives and tranquilizers can slow regular brain activity, which could impact the results of a neurological scan. If you require a sedative for anxiety, discuss this with your doctor beforehand, as a mild, approved option might be available.

Following a low-carbohydrate, high-protein, and high-fat diet for a period before a scan ensures that your body's cells are primed to absorb the FDG tracer, improving the quality of the scan images. This minimizes the amount of natural glucose in your body that would compete with the FDG tracer.