Medications
Blood-Thinner Injections (Heparin) Before Surgery
Low-molecular-weight heparin injections thin your blood to stop dangerous clots, so the timing of your last dose before surgery really matters. Because the plan depends on the exact dose and why you take it, this is one medicine you should never adjust on your own.
Quick answer
Why it matters
Heparin injections are given for a reason — usually to treat or prevent a blood clot (such as a DVT or pulmonary embolism), to protect an irregular heartbeat or a heart valve, or to "bridge" you while a longer-acting tablet like warfarin is paused before surgery. Stopping at the wrong moment can let a clot form; leaving too much in your system raises the risk of serious bleeding during the operation. The whole game is timing the last dose so the blood-thinning has safely worn off as the surgery begins, then restarting once bleeding risk has settled.
For a full treatment dose, the active effect of low-molecular-weight heparin lasts many hours, and it clears more slowly if your kidneys work less well. That is why teams usually time the last treatment dose around 24 hours before surgery rather than on the morning of the operation — and may extend that gap further if your kidney function is reduced. A small once-daily preventive dose behaves differently, but is still usually given no later than about 12 hours before surgery, especially if a spinal or epidural is planned.
"Bridging" is genuinely individualised — the schedule is built around your specific clot risk, the type of surgery, and the medicine you normally take. Two people on the same injection can have completely different stop-and-restart plans. This is also why which medicines to stop before surgery is never a one-size-fits-all list, and why your written instructions override any general advice, including this page.
Do not stop without instruction
What to ask your doctor
- What is the exact date and time of my last injection before surgery, and the dose?
- Am I on a treatment dose or a small preventive dose — and does that change the timing?
- I'm having a spinal or epidural — how long must the gap be after my last injection, and when can any epidural catheter be safely removed?
- When do I restart my injections (or my usual tablet) after the operation?
Red flags — call your team
Red flags — call your team
- New or worsening leg swelling, pain, redness, or sudden chest pain or breathlessness — possible clot — seek urgent medical help.
- Unusual bleeding or bruising, blood in urine or stool, black tarry stools, or bleeding that won't stop from the injection sites.
- After a spinal or epidural: new numbness or weakness in the legs, trouble passing urine, or severe back pain — possible bleeding around the spine — seek emergency care immediately.
- You realise you took the wrong dose, missed a dose, or are unsure of your timing before surgery — call your team straight away, do not just proceed.
References
- General anesthesia and anticoagulation guidance (e.g. ASRA 2018 regional anesthesia and anticoagulation guidelines, and ACC/ASH/ACCP perioperative anticoagulation and bridging recommendations) advises individualised timing of low-molecular-weight heparin around surgery: typically a last full treatment dose about 24 hours before surgery, and a defined interval before neuraxial (spinal/epidural) procedures — commonly at least 12 hours after a prophylactic dose and 24 hours after a treatment dose, longer with renal impairment. An epidural catheter should not be removed or a next dose given until a similar safe interval has passed. This page is general education and is not a substitute for assessment of your own clot and bleeding risk.
- Your surgical, anesthesia, and anticoagulation team's specific written instruction always takes precedence over any general guidance here. If anything conflicts, follow their plan and contact them with questions.
Frequently asked questions
Should I take my heparin injection the morning of surgery?
Usually no for a full treatment dose — the last treatment dose is typically given around 24 hours before surgery (and a twice-daily evening dose the night before is often reduced or skipped), so the blood-thinning has safely worn off. A small preventive dose is usually given no later than about 12 hours before and may be handled differently. Never decide this yourself: follow the exact time and dose your surgical or anticoagulation team has written for you, and ask if you are unsure.
What is "bridging" and why am I on injections?
Bridging means using short-acting heparin injections to cover the gap while a longer-acting tablet like warfarin is paused before surgery. Because warfarin takes days to wear off and restart, injections keep you protected from clots during that window. The schedule is built around your personal clot risk and surgery type, so it is highly individual — never copy someone else's plan or adjust your own.
How long before a spinal or epidural must I stop my heparin?
A specific, defined gap is required after your last low-molecular-weight heparin dose before a spinal or epidural can be placed safely, because doing it too soon raises the risk of bleeding around the spine. As a guide, this is commonly at least 12 hours after a preventive (prophylactic) dose and at least 24 hours after a full treatment dose, and longer if your kidney function is reduced. The same care applies to removing an epidural catheter afterwards. Your anesthesia team will set and confirm this timing — tell them exactly when your last injection was.
Does kidney function change the timing?
Yes. Low-molecular-weight heparin is cleared by the kidneys, so if your kidney function is reduced the medicine stays active longer. Your team may extend the gap before surgery, lower the dose, or choose a different approach to avoid excess blood-thinning at the time of your operation. Make sure they know about any kidney problems and any recent blood-test results so your last-dose timing is set correctly.