What medications should be avoided before a Positron Emission Tomography (PET) scan?

Medications to Avoid Before a PET Scan

Metformin should be discontinued at the time of a PET/CT procedure with intravenous contrast and withheld for 48 hours after the procedure to prevent contrast-induced nephropathy. 1

Glucose Management Before PET Scans

Proper preparation is critical for high-quality PET imaging, with particular attention to medications that may affect glucose metabolism:

• Oral hypoglycemic agents:

  • Metformin must be discontinued if IV contrast is being used during the PET/CT scan
  • Other oral diabetes medications can generally be continued, but timing may need adjustment 1

• Insulin management:

  • Rapid-acting insulin should not be administered within 4 hours of FDG injection
  • Regular/short-acting insulin should be avoided within 4-6 hours
  • Intermediate and long-acting insulin can affect scan quality but are typically continued with schedule adjustments 1

• Blood glucose considerations:

  • Ideal blood glucose level should be <200 mg/dL before the scan
  • High blood glucose levels compete with FDG uptake and can compromise image quality 1

Medications Affecting Myocardial Uptake

For cardiac PET scans or evaluations of lesions near the heart:

• A low-carbohydrate diet for 24 hours prior to the scan is recommended
• Consider avoiding medications that alter myocardial glucose metabolism if cardiac evaluation is needed 1

Sedatives and Muscle Relaxants

• Routine use of sedatives is not recommended for most adult patients
• Sedatives may be considered in specific circumstances:
  • For tumors in the head and neck region to reduce muscle uptake
  • For claustrophobic patients
  • Examples include diazepam (5 mg IV, 10 minutes before FDG) or propranolol (80 mg orally, 2 hours before FDG) to reduce brown fat uptake 1

Medications Containing Glucose

Interestingly, research shows that medications containing small amounts of glucose (up to several grams) do not significantly affect PET image quality and can generally be continued 2.

Special Considerations for Contrast-Enhanced PET/CT

If intravenous contrast will be used during the PET/CT:

• Metformin management:

  • Discontinue at the time of procedure
  • Withhold for 48 hours after procedure
  • Consider alternative glucose-controlling medication during this period 1

• Nephrotoxic medications:

  • Temporarily suspend other nephrotoxic medications in high-risk patients 3
  • High-risk patients include those with:
    • Pre-existing renal dysfunction
    • Diabetes mellitus
    • Advanced age (>70 years)
    • Congestive heart failure
    • Dehydration 3

Practical Recommendations

1. Before scheduling:

   • Inform the nuclear medicine department about all current medications
   • Disclose any history of diabetes or kidney problems

2. Preparation timeline:

   • 24 hours before: Consider low-carbohydrate diet if cardiac evaluation needed
   • 4-6 hours before: Begin fasting (water only)
   • If diabetic: Coordinate with your physician regarding insulin/medication adjustments

3. Medication adjustments:

   • Metformin: Stop if contrast will be used
   • Insulin: Coordinate timing with nuclear medicine department
   • Most other medications: Can be taken with water

Common Pitfalls to Avoid

• Failing to disclose diabetes status or medication list to imaging facility
• Taking glucose-containing beverages (including "sugar-free" ones with hidden carbohydrates)
• Exercising immediately before the scan (increases muscle uptake)
• Assuming all diabetes medications need to be stopped (only metformin with contrast)

Remember that while certain medications may need to be temporarily discontinued for optimal imaging, this should always be done in consultation with both the ordering physician and the nuclear medicine department to ensure patient safety.