Ketamine does work as a painkiller, though it wasn’t originally designed for that purpose. It’s FDA-approved as an anesthetic, meaning it was developed to make people unconscious during surgery. Its use for pain relief at lower, “sub-anesthetic” doses is technically off-label, but it’s widely used in hospitals, emergency departments, and specialty pain clinics for exactly that reason.
How Ketamine Blocks Pain
Most painkillers work by either reducing inflammation (like ibuprofen) or dulling pain signals at opioid receptors in the brain (like morphine). Ketamine takes a completely different route. It blocks a receptor in the brain and spinal cord called the NMDA receptor, which normally allows calcium to flow into nerve cells when activated by the chemical messenger glutamate. When calcium floods in, neurons fire and amplify pain signals. Ketamine physically plugs the channel inside this receptor, preventing that cascade.
What makes this especially useful is that NMDA receptors play a central role in something called central sensitization, a process where the nervous system essentially turns up its own volume knob on pain. Over time, this can make ordinary sensations feel painful (a light touch on the skin becoming agonizing, for instance) or cause pain to spread beyond the original injury site. By blocking NMDA receptors, ketamine doesn’t just reduce pain in the moment. It can also dial back that amplified sensitivity, which is why it’s particularly valuable for certain chronic pain conditions where the nervous system has become hypersensitive.
Ketamine for Acute Pain
In emergency rooms and operating rooms, ketamine is used at doses far below what would cause unconsciousness. A typical sub-anesthetic dose ranges from 0.1 to 0.5 mg per kilogram of body weight as an initial injection, with continuous infusions running at 0.1 to 0.2 mg per kilogram per hour. For a 70 kg (154 lb) person, that initial dose might be somewhere between 7 and 35 mg, compared to the much larger doses used for full anesthesia.
One of its biggest advantages in acute settings is that it can reduce the amount of opioid painkillers a patient needs after surgery. In a study of patients recovering from spinal fusion surgery, those receiving a ketamine infusion used roughly 40% less fentanyl over the 48 hours after their operation compared to those who received only standard pain control. This “opioid-sparing” effect matters because less opioid use means fewer side effects like nausea, constipation, and respiratory depression, and potentially lower risk of dependence.
Ketamine is also valuable in trauma and emergency settings where patients arrive in severe pain. It can be given to people who don’t tolerate opioids well or whose pain isn’t responding adequately to standard medications.
Ketamine for Chronic Pain
The chronic pain applications are where ketamine gets more experimental and more interesting. Clinicians have used ketamine infusions for conditions that are notoriously difficult to treat: complex regional pain syndrome (CRPS), neuropathic pain, phantom limb pain, and fibromyalgia. These are all conditions where central sensitization plays a major role, which is precisely the mechanism ketamine targets.
Treatment protocols vary widely. Some involve a single infusion, while others use daily sessions over 4 to 10 days. For CRPS, published protocols have included 4-hour daily infusions for 10 consecutive days. For fibromyalgia, one documented approach used 10 initial daily infusions of one hour each (weekdays only), followed by maintenance sessions of two infusions per month. The total amount of ketamine administered across these protocols ranges enormously, from about 20 mg to 3,000 mg total, depending on the condition and the clinic’s approach.
Results are promising but inconsistent. Some patients experience weeks or months of significant pain relief after a series of infusions. Others see minimal benefit. There’s no standardized protocol yet, and insurance coverage for ketamine pain infusions remains limited since the FDA has not approved ketamine specifically for pain management.
How It’s Given and Why That Matters
The way ketamine enters your body dramatically affects how well it works for pain. Intravenous (IV) delivery puts the full dose directly into the bloodstream, making it the most reliable method. Intramuscular injection (a shot into muscle tissue) absorbs rapidly with 93% of the drug reaching the bloodstream, and in studies it produced a noticeable rise in pain threshold lasting 15 to 60 minutes.
Oral ketamine is a different story. Only about 17% of a swallowed dose actually makes it into the bloodstream because the liver breaks down most of it before it can circulate. In controlled testing, oral ketamine produced little to no measurable increase in pain tolerance. Some clinics still prescribe oral ketamine lozenges or tablets for chronic pain, often as a take-home option between infusion appointments, but the pain-relieving effects are considerably weaker than IV or intramuscular delivery. Intranasal (nasal spray) ketamine falls somewhere in between, with better absorption than oral but less predictable than IV.
Side Effects at Pain-Relieving Doses
Even at the low doses used for pain, ketamine produces noticeable psychological effects. The most characteristic is dissociation: a feeling of detachment from your body or surroundings, sometimes accompanied by distorted vision or hearing. This isn’t the same as being “knocked out.” Patients typically remain awake and aware but may feel floaty, disconnected, or as though things around them aren’t quite real. These effects generally resolve within an hour or two after the infusion stops.
Psychosensory side effects tend to increase at doses above 0.3 mg per kilogram, which is why clinicians treating awake patients generally stay below that threshold for single doses. Other common side effects include nausea, dizziness, and a temporary increase in heart rate and blood pressure. The blood pressure rise is why ketamine is typically avoided in people with uncontrolled hypertension or cardiovascular instability. It’s also not used in people with a history of psychotic disorders, active substance use disorders, or during pregnancy.
Interestingly, research has shown that the dissociative effects and the pain-relieving effects operate through separate mechanisms. You don’t need to feel dissociated for ketamine to reduce your pain, and feeling dissociated doesn’t necessarily mean you’re getting better pain relief.
How Ketamine Compares to Opioids
Ketamine and opioids reduce pain through entirely different pathways, which is why they work well together and why ketamine can sometimes succeed where opioids fail. Opioids are strong for acute, nociceptive pain (the straightforward kind caused by tissue damage), but they’re less effective against neuropathic pain, where damaged nerves themselves generate pain signals. Ketamine, by blocking NMDA receptors and reducing central sensitization, targets exactly that type of pain.
Ketamine also lacks some of opioids’ most dangerous properties. It does not suppress breathing at analgesic doses, which is the primary way opioid overdoses become fatal. It does not cause the same degree of physical dependence with short-term use, and it does not slow gut motility (meaning no constipation). These advantages make it a useful alternative or complement, particularly for patients who are opioid-tolerant and getting diminishing returns from their current medications.
That said, ketamine is not a simple substitute for opioids. It requires medical supervision for IV administration, produces dissociative side effects that many patients find unpleasant, and its long-term safety with repeated use is less well-established than that of many conventional pain medications. Chronic or frequent use carries risks including bladder damage and cognitive effects that are still being studied.