When Is Lower Back Pain Serious? 8 Warning Signs

The short answer: Most lower back pain is not caused by anything serious and improves with time. It becomes serious when it comes with numbness around your genitals or bottom, new bladder or bowel problems, leg weakness, fever, unexplained weight loss, or follows a serious injury. These signs need prompt medical care.

• Most lower back pain is "non-specific", meaning no dangerous cause is found. The World Health Organization (WHO) puts it at about 90% of cases [1].
• A short list of warning signs, often called "red flags", means you should not wait it out.
• Numbness around your genitals or bottom, or new bladder or bowel trouble, is an emergency.
• One red flag on its own is usually not a sign of anything serious. Doctors look at the whole picture [5].
• If back pain is not improving after a few weeks, see a doctor even if you have no red flags [3][4].

Almost certainly not. Back pain is frightening, especially when it comes on suddenly, but serious causes are rare.

The WHO estimates that about 90% of low back pain is non-specific, with no clear underlying disease or tissue damage [1]. The NHS says the cause is often unclear and that back pain frequently improves on its own. A pulled muscle or strain is a common reason [3].

A study of 1,172 people who saw a doctor in Sydney, Australia, for sudden-onset low back pain found serious pathology in 11 of them (0.9%). Eight were vertebral fractures, meaning broken bones in the spine [6]. That is one study in one country, but it fits the wider picture: serious causes are the exception.

Still, a few causes are serious enough that you should know the signs. That is what the rest of this guide is about.

Serious back pain usually comes with other symptoms, not just pain in the back. These are the eight to watch for. Where I say what a sign "can point to", that is the doctor's reasoning, not a diagnosis.

The NHS also treats chest pain with back pain, and changes in genital feeling during sex or trouble getting or keeping an erection, as emergency symptoms [3].

Emergency: get help now

Signs 1 to 4 are the ones where waiting can be harmful. Numbness around the genitals or anus, new bladder or bowel changes, and symptoms in both legs together are the classic signs doctors worry about at the base of the spine, sometimes called cauda equina syndrome.

Call your local emergency number or go to the nearest emergency department. Common numbers are 999 in the UK, 911 in the US and Canada, 112 across the EU and 000 in Australia. The NHS adds: do not drive yourself. Ask someone to take you, or call an ambulance [3].

Urgent: get seen today

Fever, feeling shivery or generally unwell, or sudden severe pain that is getting worse quickly, should be assessed the same day. In the UK, call NHS 111 or ask for an urgent GP appointment [3]. Elsewhere, use an urgent care clinic, a same-day doctor or a nurse advice line. Mayo Clinic, in the US, lists fever with back pain among the signs to seek immediate care for [4].

See a doctor soon: book an appointment

Unexplained weight loss, pain that is worse at night or does not ease with rest, pain that spreads down a leg below the knee, or weakness, numbness or tingling in one leg should be checked by a doctor [3][4]. The NHS also lists a lump or swelling, a change in the shape of your back, pain between the shoulder blades, and pain that gets worse when you cough, sneeze or open your bowels [3].

No. A red flag is a reason to get checked, not a diagnosis. Many people with back pain have at least one red flag, and most of them have nothing serious.

A BMJ systematic review (Downie and colleagues, 2013) looked at 14 studies covering 53 red flags. Only a small number had good evidence that they help predict a broken spine bone or cancer. For fracture, the strongest were severe trauma, a bruise or graze on the back, long-term use of corticosteroid medicine, and having several red flags at once. For cancer, a past history of cancer was the one that clearly raised the chances. Older age, unexplained weight loss and pain that had not improved after a month each carried a probability of cancer below 3% [5].

The Sydney study agreed: some red flags produced many false alarms when used alone [6].

What this means: do not panic over a single sign, and do not ignore a cluster of them. Tell your doctor everything, including a past cancer, steroid use, a recent fall, or weight you have lost without trying. They weigh the combination.

Most lower back pain comes from strained muscles or ligaments, often after heavy lifting or an awkward twist. You might call it a pulled muscle or a "back spasm". Other common causes include a slipped disc (a bulging disc that can press on a nerve), arthritis and narrowing of the spinal canal. Less often, back pain comes from a broken bone, infection, cancer, or an inflammatory condition such as ankylosing spondylitis [3][4].

Back pain can also come from organs near the spine. The WHO gives kidney problems and an aortic aneurysm (a bulge in the main blood vessel) as examples [1]. That is another reason sudden, severe pain needs prompt assessment.

Sometimes the cause stays unclear. Mayo Clinic notes that disc problems often show up on scans even in people who have no pain, so a scan does not always explain your symptoms [4]. If pain travels down your leg, see our guide to how long sciatica lasts.

Stay as active as you can. The NHS advises keeping up daily activities, and says walking, swimming, yoga and pilates may help. Avoid staying in bed for long periods [3].

Other self-care steps from the NHS [3]:

• Heat or ice: wrap a heat pack or hot water bottle in a tea towel for stiffness or spasm. Use an ice pack or frozen peas, wrapped, to ease pain and swelling.
• Painkillers: anti-inflammatories such as ibuprofen may help if you can take them. Paracetamol alone is not recommended for back pain. Ask a pharmacist or doctor which suits you.
• Exercises and stretches: try them, but stop if pain gets worse.

The WHO's 2023 guideline on long-lasting (chronic) low back pain recommends education, exercise, and some physical and psychological therapies, combined rather than used alone. It advises against lumbar braces and belts, traction and opioid painkillers for most people [2]. It covers pain lasting more than three months with no underlying disease, so it does not apply to a new back injury.

Two gentle exercises are a sensible starting point. They are common first steps, not a treatment programme:

1. Bird-dog. 

On your hands and knees, slowly reach one arm forward and the opposite leg back, keeping your back flat. Hold for 3 to 5 seconds. Do 5 to 8 repetitions each side. See our bird-dog exercise.

1. Dead bug. 

Lie on your back with your arms pointing up and your knees bent over your hips. Slowly lower one arm and the opposite leg towards the floor, then return. Keep your lower back gently against the floor. Do 5 to 8 repetitions each side. See our dead bug exercise.

Stop if your pain gets worse, spreads down a leg, or you feel tingling or numbness. If any warning sign from the table appears, get medical help instead of exercising.

Kai, Loosn's AI physio coach, can help you pace gentle movement around your pain day by day. It does not replace an assessment if your symptoms are severe, getting worse or unusual.

If you are wondering about rest, see our guide to whether bed rest is bad for back pain.

If you have no warning signs, a few weeks is the usual guide. The NHS says to see a doctor if pain has not improved after a few weeks of home treatment, or if it disrupts your daily life or you are struggling to cope [3]. Mayo Clinic gives the same guide: back pain lasting more than a few weeks, or severe and not easing with rest [4].

MedlinePlus, from the US National Library of Medicine, is more cautious: if back pain is severe or has not improved after three days, call your health care provider, and seek care for pain that follows an injury [7]. If you are unsure, the cautious option is to ask.

This box is for clinician review before publication.

Emergency: call your local emergency number or go to the emergency department if back pain comes with [3][4]:

• numbness or loss of feeling around your genitals or anus
• new bladder or bowel problems, such as trouble peeing or losing control
• pain, tingling, weakness or numbness in both legs
• chest pain
• a start after a serious accident, fall or blow

Urgent: get seen today (UK: call 111 or ask for an urgent GP appointment) if back pain comes with [3][4]:

• fever, shivering, or feeling hot, cold or generally unwell
• sudden severe pain, or pain that is getting worse quickly

See a doctor soon if you have [3][4]:

• weight loss you cannot explain
• pain that is worse at night or does not ease with rest
• weakness, numbness or tingling in one leg, or leg weakness that is getting worse
• pain that has not improved after a few weeks of home treatment

Loosn is not a substitute for assessment when symptoms are severe, getting worse or unusual.

Is back pain at night serious? 

Not always, but it deserves a check. The NHS lists pain that is worse at night or does not ease with rest among the reasons to see a doctor [3]. Mayo Clinic lists severe pain that does not improve with rest [4]. Book an appointment, and go sooner if you also have a fever or weight loss.

Should I go to the emergency room for back pain? 

Go if you have numbness around your genitals or anus, new bladder or bowel problems, symptoms in both legs, chest pain, or pain after a serious accident [3]. For ordinary back pain without these signs, a doctor's appointment or a pharmacist is usually enough.

Can back pain be a sign of cancer? 

Rarely. The NHS lists cancer as a rare cause of back pain [3]. In a BMJ review, a past history of cancer was the clearest warning sign, and weight loss alone carried a probability below 3% [5]. Tell your doctor about any past cancer or weight loss.

Does back pain that goes down my leg mean something serious? 

Usually not. Mayo Clinic advises seeing a doctor for pain that spreads down one or both legs, especially below the knee [4]. Weakness, numbness or tingling in both legs is an emergency [3]. See our guide to how long sciatica lasts.

Do I need an MRI or X-ray for back pain? 

Often not. Mayo Clinic notes that disc problems commonly show on scans in people without pain [4]. The BMJ review authors argued that scanning everyone with a single red flag would lead to unnecessary imaging, since many people have at least one [5]. Your doctor decides based on the whole picture.

Is walking good for back pain? 

Yes, for most people. The NHS says staying active, including walking, may help, and advises against long periods in bed [3]. Stop if walking makes pain worse or sends it down a leg. See our guide to walking with sciatica.

If your back pain has no warning signs, the Loosn app gives you gentle exercises and a daily plan from Kai, Loosn's AI physio coach. Download the Loosn app.