If your doctor has advised a PET CT scan, it is completely natural to have one question at the top of your mind: what are the side effects of a PET scan? Is the radioactive injection harmful? Will you feel unwell afterwards? Can you go home to your children the same day? This guide answers every one of these questions in plain language, based on established nuclear medicine safety guidelines and the day-to-day clinical experience of the nuclear medicine team at Neurad Diagnostic & Healthcare LLP in Paschim Vihar, Delhi.
Quick answer: A PET scan is a safe, non-invasive diagnostic procedure for the vast majority of patients. Serious side effects are rare. The most common effects are minor — slight discomfort or redness at the injection site, a metallic taste for a short while, or mild nausea. The radiation dose from a single PET CT scan is low-level, temporary, and leaves the body within about a day, mainly through urine. Most patients resume completely normal activities the same day, with a few simple precautions explained below.
How a PET Scan Works — and Why Side Effects Are Limited
To understand PET scan side effects, it helps to understand what actually happens during the test. A PET (Positron Emission Tomography) scan uses a very small, medically calculated amount of a radioactive tracer — most commonly FDG (18F-Fluorodeoxyglucose), a modified form of glucose. The tracer is injected into a vein, circulates through the body, and accumulates in metabolically active tissues. The scanner then detects the signal emitted by the tracer and builds detailed images of how your organs and tissues are functioning.
Two facts explain why PET scan side effects are limited for most patients:
- The tracer dose is tiny. The amount of FDG or Gallium-68 tracer injected is measured in micrograms — far too small to have any pharmacological (drug-like) effect on the body. It does not sedate you, does not alter your heart rate or blood pressure, and does not interact with most medications.
- The radioactivity is short-lived. Fluorine-18 has a half-life of about 110 minutes, and Gallium-68 only about 68 minutes. This means the radioactivity decays rapidly on its own, and the remainder is flushed out through urine, typically within 24 hours for F-18 tracers and even faster for Ga-68 tracers.
Professional bodies including the Society of Nuclear Medicine and Molecular Imaging (SNMMI) and the International Atomic Energy Agency (IAEA) classify PET CT as a low-risk diagnostic procedure whose benefits, when medically indicated, outweigh the small radiation exposure involved.
Common Mild PET Scan Side Effects
These are the side effects patients most often ask about. All of them are temporary and resolve on their own without treatment.
1. Injection Site Discomfort
The tracer is given through a standard intravenous cannula, similar to a blood test needle. Some patients notice mild pain, redness, or a small bruise at the injection site. This usually settles within a day or two. Rarely, a small amount of tracer can leak under the skin (extravasation), causing temporary swelling — this is not dangerous and resolves as the tracer decays.
2. Metallic or Unusual Taste
A brief metallic taste in the mouth immediately after the injection is reported by some patients. It typically fades within minutes and needs no treatment.
3. Mild Nausea
Because you fast for 4–6 hours before an FDG PET scan, some patients feel mildly nauseous or lightheaded — often from the fasting itself rather than the tracer. Drinking water (which is allowed and encouraged) and resting during the uptake period usually helps. Vomiting caused by the tracer alone is uncommon.
4. Headache or Dizziness
Occasional mild headache or dizziness can occur, again frequently linked to fasting, anxiety, or lying still for the scan duration. These symptoms pass quickly once you eat and hydrate after the scan.
5. Feeling Cold During the Uptake Period
Before an FDG scan you rest quietly for 45–60 minutes while the tracer distributes. Rooms are kept cool, and lying still can make some patients feel cold. Inform the technologist — a blanket is always available. Staying warm actually improves image quality by reducing brown fat uptake.
6. Anxiety and Claustrophobia
The PET CT scanner is an open, ring-shaped machine — it is open at both ends and is significantly less enclosed than a conventional MRI tunnel. Even so, patients who are anxious in medical settings may feel uneasy. At Neurad Diagnostic, the technologist remains in voice contact with you throughout the scan, and you can signal at any time. True claustrophobic reactions during PET CT are uncommon.
PET Scan Radiation Exposure: How Much Is It Really?
Radiation is the concern behind most searches for PET scan side effects, so let us address it directly with numbers rather than vague reassurance.
Radiation dose is measured in millisieverts (mSv). For context, every person on Earth receives natural background radiation from soil, cosmic rays, and food — roughly 2–3 mSv every year, simply by living.
| Source of Radiation | Approximate Effective Dose |
|---|---|
| Chest X-ray | ~0.1 mSv |
| Natural background radiation (per year) | ~2–3 mSv |
| Diagnostic CT chest/abdomen | ~5–10 mSv |
| PET component (FDG tracer alone) | ~4–7 mSv |
| Combined FDG PET CT (low-dose CT protocol) | ~8–14 mSv (varies by protocol and body size) |
| Ga-68 PET tracers (PSMA, DOTANOC, FAPI) | Generally lower tracer dose than F-18 FDG |
Key points about PET scan radiation:
- It is a one-time, short-lived exposure — not a continuous or cumulative implant. The tracer decays and is excreted within hours to a day.
- Doses are individually calculated. The injected activity is adjusted for your body weight, following the ALARA principle (As Low As Reasonably Achievable) used in nuclear medicine worldwide.
- The theoretical long-term risk is very small. Radiation at diagnostic levels carries a small statistical risk, which is why PET scans are only performed when there is a genuine medical indication — typically cancer diagnosis, staging, or treatment monitoring, where the information gained directly shapes treatment decisions.
- Modern scanners reduce dose. Current-generation PET CT systems use low-dose CT protocols for attenuation correction, which keeps the CT contribution considerably lower than a full diagnostic CT.
In practical terms: for a patient whose oncologist needs to know whether cancer has spread, the diagnostic value of the scan is substantial, while the radiation exposure is comparable to a few years of natural background radiation, delivered once.
Contrast-Related Side Effects (When IV Contrast Is Used)
Some PET CT protocols include an iodinated contrast-enhanced CT for better anatomical detail. The contrast dye — not the PET tracer — is responsible for a separate, well-understood set of possible effects:
- Warm flushing sensation spreading through the body during injection — normal and brief.
- Mild reactions such as itching, sneezing, or a light rash — uncommon and usually self-limiting.
- Allergic reactions. Moderate reactions are rare; severe reactions (anaphylaxis) are very rare. Imaging centres performing contrast studies maintain emergency medications and trained staff for exactly this reason.
- Kidney considerations. In patients with significantly reduced kidney function, iodinated contrast is used cautiously or avoided. This is why you may be asked for a recent serum creatinine report before a contrast-enhanced protocol.
Important: if your PET CT is performed without IV contrast (a common protocol), none of these contrast-related effects apply to you. Always inform the team about any previous contrast reaction, asthma, or kidney disease — the protocol can be adjusted accordingly.
Rare Side Effects of PET Scans
For completeness, these reactions are documented in medical literature but are genuinely uncommon:
- Allergic reaction to the tracer itself. True allergy to FDG or Ga-68 peptide tracers is exceptionally rare because the injected chemical mass is minute.
- Vasovagal episode (feeling faint) around the time of injection — related to needle anxiety rather than the tracer, and managed by lying down and hydration.
- Hypoglycemia-related symptoms in diabetic patients whose medication and fasting schedule were not adjusted — preventable with proper pre-scan instructions (see below).
PET Scan Safety in Special Situations
Pregnancy
PET scans are generally avoided during pregnancy because the developing fetus is more sensitive to radiation. If you are pregnant, could be pregnant, or have missed a period, tell the team before your appointment. If imaging is medically essential during pregnancy, the referring doctor and nuclear medicine physician will weigh alternatives and, where unavoidable, use dose-minimizing protocols. A routine pregnancy check is part of responsible pre-scan screening for women of childbearing age.
Breastfeeding
Small amounts of tracer can appear in breast milk, and — more significantly — close cuddling exposes the infant to external radiation from the mother’s body while the tracer is active. Guidance depends on the specific tracer, so follow the personalised instructions of the nuclear medicine physician. A common practical approach for FDG is to feed the baby just before the injection, then avoid prolonged close contact for several hours afterwards, using expressed milk if needed. Do not rely on generic internet advice here — ask the team on the day of your scan.
Diabetes
High blood glucose competes with FDG and degrades image quality, so blood sugar is checked before every FDG PET scan. Diabetic patients receive specific instructions about the timing of insulin or oral medication relative to the fasting period. Followed correctly, diabetes does not increase the side-effect risk of the scan itself — it only requires schedule planning. Bring your glucometer and medication with you.
Kidney Disease
The FDG tracer itself does not damage kidneys. Kidney function mainly matters when iodinated CT contrast is planned, as described above. Patients on dialysis can undergo PET imaging with coordinated scheduling.
Children and Elderly Patients
When PET imaging is medically indicated in children, tracer activity is scaled down by body weight according to paediatric dosage guidelines. Elderly patients tolerate PET scans well; the main practical considerations are comfortable positioning and hydration.
Precautions After a PET Scan: What To Do for the Next 24 Hours
This is one of the most searched questions — what precautions should be taken after a PET scan? Here is the practical checklist we give patients at Neurad Diagnostic:
- Hydrate well. Drink plenty of water for the rest of the day. The tracer is excreted through urine, and good hydration speeds up clearance.
- Use the toilet normally, with basic hygiene. Flush normally and wash hands. Male patients may be advised to urinate sitting down for the first few hours to avoid splashes.
- Limit prolonged close contact with pregnant women and small children for several hours. You are not “dangerous” to others, but as a sensible precaution, avoid extended cuddling, lap-sitting, or sleeping next to a pregnant woman or infant on the day of the scan. Normal conversation, sharing a room, and brief contact are fine. By the next morning, no restrictions remain for F-18 tracers.
- Resume eating immediately. There is no dietary restriction after the scan. In fact, eating helps you recover from the fasting period.
- Resume normal activities. You can drive, work, and exercise the same day unless you received sedation (rare) or your doctor advises otherwise.
- Airport and metro radiation detectors. Sensitive radiation portals (airports, some metro stations, border crossings) can detect residual tracer for roughly 24 hours or slightly longer. If you plan to travel immediately after your scan, ask the centre for a scan certificate stating the isotope, dose, and date — this is a routine document.
Do Side Effects Differ Between Tracer Types?
Not all PET scans use the same tracer, and patients at Neurad Diagnostic often undergo specialised Gallium-68 studies. The safety profile is broadly similar across tracers, with small practical differences:
- FDG (18F) PET CT — the standard tracer for whole body PET CT scans. Requires 4–6 hours fasting and blood glucose control. Side effects as described throughout this article.
- 68Ga PSMA PET CT — used for prostate cancer imaging. No fasting required for the tracer itself. The shorter half-life of Ga-68 means radioactivity clears even faster. Mild dry mouth is theoretically possible due to salivary gland uptake but is rarely noticeable at diagnostic doses.
- 68Ga DOTANOC PET CT — used for neuroendocrine tumour imaging. Patients on long-acting somatostatin analogue are scheduled relative to their injection cycle; the tracer itself is well tolerated.
- 68Ga FAPI, Exendin and Trivehexin PET CT — specialised tracers from the in-house 68Ga Gallium Generator programme at Neurad Diagnostic. These peptide tracers share the favourable Ga-68 safety profile. Exendin scans involve supervised glucose monitoring in insulinoma patients — a planned clinical protocol, not a side effect.
- FDOPA and F-Choline PET CT — F-18 based specialised tracers with preparation requirements explained at booking; the general safety profile matches FDG.
When Should You Contact a Doctor After a PET Scan?
Serious problems after a PET scan are rare, but contact the imaging centre or your doctor promptly if you notice any of the following:
- Spreading redness, significant swelling, or increasing pain at the injection site after 48 hours
- Hives, facial swelling, wheezing, or difficulty breathing (possible allergic reaction — seek immediate care)
- Persistent vomiting preventing you from keeping fluids down
- In diabetic patients: symptoms of significant low blood sugar such as sweating, tremors, or confusion
PET Scan Side Effects: Myths vs Facts
Myth: “The radioactive injection stays in the body for weeks.”
Fact: F-18 tracers largely decay and clear within about 24 hours; Ga-68 tracers even faster. Hydration accelerates clearance.
Myth: “A PET scan makes you permanently radioactive or infertile.”
Fact: The exposure is temporary and low-level. Diagnostic PET doses are far below thresholds associated with fertility effects.
Myth: “PET scan side effects include hair loss.”
Fact: Hair loss is a side effect of certain chemotherapy and high-dose radiotherapy — not of diagnostic PET imaging. Patients sometimes conflate the two because PET scans occur during cancer treatment journeys.
Myth: “You must isolate from your family for days.”
Fact: Only a few hours of sensible distance from pregnant women and infants is advised on the scan day itself. Normal family life resumes by the next day.
Myth: “The scan is painful.”
Fact: The only needle involved is the initial IV cannula. The scan itself is painless — you simply lie still while the table moves through the open scanner ring.
How Neurad Diagnostic Minimizes PET Scan Risks
At Neurad Diagnostic & Healthcare LLP, Paschim Vihar, safety protocols are built into every step of the PET CT workflow:
- Pre-scan screening covering pregnancy status, breastfeeding, diabetes, kidney function (for contrast protocols), allergies, and current medications
- Weight-based tracer dosing following ALARA principles, so no patient receives more activity than the study requires
- Blood glucose verification before every FDG study to protect both image quality and diabetic patients
- Supervision by qualified nuclear medicine physicians — Dr. Swagat Dash (MBBS, DNB – Nuclear Medicine) and Dr. Pradeep Chaurasiya (MBBS, DNB – Nuclear Medicine) — throughout tracer handling, injection, and reporting
- Clear written after-care instructions handed to every patient, including travel certificates on request
- Reports within 24 hours, so the diagnostic benefit of the scan reaches your treating doctor quickly
If you have questions about an upcoming scan — or want to understand what affects PET CT scan cost in Delhi — the team is available on +91 8368225620 and +91 7011968879.
Frequently Asked Questions About PET Scan Side Effects
1. Is a PET scan harmful to the body?
For medically indicated use, no. A PET scan involves a low, one-time radiation exposure and a tracer dose too small to have drug-like effects. Serious reactions are rare, and the diagnostic information typically has a direct impact on treatment decisions.
2. How long does the radioactivity stay in the body after a PET scan?
For F-18 FDG scans, most radioactivity decays or is excreted within about 24 hours. For Gallium-68 scans (PSMA, DOTANOC, FAPI), clearance is even faster because of the 68-minute half-life. Drinking water speeds up excretion through urine.
3. Can I be around my family after a PET scan?
Yes. The only advice is to avoid prolonged close contact — such as extended cuddling or co-sleeping — with pregnant women and infants for several hours on the day of the scan. From the next day onwards, no restrictions apply.
4. What are the most common side effects of the PET scan injection?
Mild injection-site discomfort or bruising, a brief metallic taste, and occasional mild nausea or headache. All are temporary and resolve without treatment.
5. Does a PET scan have more radiation than a CT scan?
A combined PET CT delivers a dose in the same general range as a diagnostic CT of the chest or abdomen, because it includes both a tracer dose and a CT component. Modern low-dose CT protocols keep the total exposure moderate.
6. Can diabetic patients safely have a PET scan?
Yes, with planning. Blood glucose is checked before the scan, and medication timing is adjusted around the fasting window. The scan itself carries no additional risk for diabetic patients when these steps are followed.
7. Is a PET scan safe during pregnancy or breastfeeding?
PET scans are generally avoided in pregnancy unless essential. Breastfeeding mothers receive tracer-specific guidance — typically feeding before the injection and avoiding prolonged close contact for several hours afterwards. Always inform the team about pregnancy or breastfeeding before the scan.
8. Will I feel sick or tired after a PET scan?
Most patients feel completely normal. Mild tiredness, when present, usually relates to fasting and the length of the appointment rather than the tracer. Eating and hydrating after the scan restores energy quickly.
9. Can airport security detect that I had a PET scan?
Yes — sensitive radiation portals can pick up residual tracer for around 24 hours or slightly longer. If you are travelling soon after your scan, request a scan certificate from the centre stating the isotope, dose, and date.
10. How can I reduce PET scan side effects?
Follow the preparation instructions exactly (fasting, hydration, medication timing), inform the team about allergies, kidney disease, pregnancy or breastfeeding, drink plenty of water after the scan, and rest if you feel lightheaded from fasting. These simple steps prevent the majority of avoidable discomfort.
Conclusion: A Low-Risk Test With High Diagnostic Value
PET scan side effects, for the great majority of patients, amount to little more than a needle prick, a short fasting window, and a day of drinking extra water. The radiation exposure is real but low, short-lived, and medically justified when the scan is properly indicated — which is precisely why PET CT is only performed on a doctor’s prescription. Understanding these facts before your appointment removes most of the anxiety that surrounds the word “radioactive.”
If you have a PET CT scan scheduled — whether a standard whole body FDG PET CT or a specialised Gallium-68 study — and you still have safety questions specific to your health condition, call Neurad Diagnostic at +91 8368225620. The nuclear medicine team will answer your questions before you book, not after.
Neurad Diagnostic & Healthcare LLP
B-3, opp. Metro Pillar No. 233, Near Paschim Vihar West Metro Station, New Multan Nagar, Paschim Vihar, New Delhi – 110056
Phone: +91 8368225620 | +91 7011968879
Website: https://neuraddiagnostic.com
About the Medical Team
Dr. Swagat Dash — MBBS, DNB (Nuclear Medicine & PET-CT Imaging)
Over a decade of clinical expertise in nuclear medicine and advanced diagnostic PET-CT imaging.
Dr. Pradeep Chaurasiya — MBBS, DNB (Nuclear Medicine, Molecular Imaging)
Several years of expertise in PET-CT scans and nuclear medicine procedures.
Dr. Nishant Thapar — MBBS, MD Radiology (High-End Diagnostic Imaging, MRI, CT)
Extensive experience in diagnostic imaging with advanced radiological equipment.
Practicing at Neurad Diagnostic & Healthcare LLP, B-3, Near Paschim Vihar West Metro Station, New Multan Nagar, Paschim Vihar, New Delhi — 110056.
Clinically reviewed by the nuclear medicine team at Neurad Diagnostic & Healthcare LLP. This content is for general information only and is not a substitute for advice from your treating doctor. Individual radiation doses, preparation steps, and after-care instructions vary by tracer and protocol — always follow the specific instructions provided at your appointment.