What Are the Side Effects of a PET-CT Scan?

Positron emission tomography–computed tomography (PET-CT) is a powerful hybrid imaging modality widely used in oncology, neurology, and cardiology for disease detection, staging, treatment planning, and response assessment. While PET-CT is generally considered safe and well tolerated, it does carry potential risks and side effects that clinicians and patients should understand.

The most significant consideration is ionizing radiation exposure. A standard FDG PET-CT scan delivers an effective dose of approximately 14–25 millisieverts (mSv), depending on protocol, patient size, and scanner technology. This represents a cumulative exposure from both the radiotracer—typically fluorine-18 fluorodeoxyglucose (¹⁸F-FDG)—and the low-dose or diagnostic CT component. Although this level falls within accepted safety limits for medically indicated studies, repeated examinations over time warrant careful justification to adhere to the ALARA principle (As Low As Reasonably Achievable).

Acute adverse reactions to ¹⁸F-FDG are exceedingly rare. Unlike iodinated CT contrast agents, FDG does not contain iodine and is not associated with allergic-type hypersensitivity reactions. No premedication is required, and no cases of anaphylaxis have been reported in the literature. Similarly, FDG has no pharmacologic activity—it is not a drug—and therefore causes no direct physiological effects such as changes in blood pressure, heart rate, or respiratory function.

Minor, transient discomfort may occur at the injection site, including mild pain, redness, or swelling—typically resolving spontaneously. Patients with diabetes require special attention: hyperglycemia can impair FDG uptake in target tissues, potentially reducing diagnostic sensitivity. Therefore, blood glucose levels are routinely checked prior to injection, and scans may be rescheduled if levels exceed institutional thresholds (commonly >150–200 mg/dL).

Pregnancy and breastfeeding represent important contraindications. PET-CT is avoided during pregnancy unless absolutely necessary, given theoretical fetal radiation risk—particularly during organogenesis. For lactating individuals, guidelines recommend temporary interruption of breastfeeding (typically 12–24 hours post-injection) and discarding expressed milk during that interval, as small amounts of FDG are excreted in breast milk.

In summary, PET-CT is a low-risk procedure with an excellent safety profile when appropriately indicated. Its benefits in guiding critical clinical decisions far outweigh its minimal risks for most patients. However, thoughtful patient selection, radiation dose optimization, and attention to metabolic and physiological variables—including glycemic control and lactation status—are essential components of safe, high-quality PET-CT practice.