Lower Back Pain on the Left Side: Common Causes and When to Worry

Most pain on the left side of the lower back comes from a muscle or joint.

It’s sore to press, it changes when you bend or twist, and it often follows something you did: a long drive, lifting a suitcase, a new gym routine. Heat helps that kind.

Your left kidney, part of your bowel and, in women, the left ovary sit close to the same area. Pain from them can feel like back pain. Before you reach for a heating pad, check which kind you have.

Signs it’s muscle or joint pain

• It gets worse or better when you move, bend, twist or change position.
• A spot is tender when you press on it. See muscle knots, heat and a massage ball.
• It started after activity, sitting for long, or sleeping awkwardly.
• You feel well otherwise: no fever, no change in your urine or bowels.

Common musculoskeletal causes

Muscle strain. The deep muscle between your lowest rib and your pelvis (quadratus lumborum) often takes the load when you lift or twist to one side. It feels like a tight, aching knot just above the hip.

Sacroiliac (SI) joint pain. Pain over the dimple at the back of the pelvis, sometimes into the buttock. Common in pregnancy, after childbirth and in runners. Standing on one leg or climbing stairs may aggravate it. If it sits further out on the side of the hip, see outer hip pain.

Disc or joint irritation in the lower spine, which can favour one side. If pain or tingling runs down the leg, a nerve may be involved: see sciatica and heat.

For all of these, heat for 15–20 minutes followed by gentle movement is a good start. See how to use a heating pad for lower back pain.

Causes that aren’t in your back

Don’t treat these with heat. See a doctor.

Possible cause	Clues
Kidney stone	Waves of severe pain in the side, spreading to the groin. You can’t get comfortable in any position. Sometimes blood in the urine.
Kidney infection	Fever, chills, burning or frequent urination, feeling unwell.
Bowel problems	Pain with constipation, bloating, change in bowel habits, blood in your stool, or fever (diverticulitis often affects the left side).
Ovarian cyst or endometriosis	Pain linked to your cycle, pain during sex, or sudden sharp one-sided pain.
Possible pregnancy	A late period with one-sided pain needs checking the same day.

Where cycle-linked pain has already been diagnosed, see heat and endometriosis and heating pad for period cramps. For gut cramping, heat for IBS cramps.

Go to A&E or call 995 if…

• The pain is sudden, severe and tearing, especially if you’re older, smoke, or have high blood pressure.
• You have high fever with back or side pain and feel very unwell.
• You have numbness around your groin, or can’t control your bladder or bowels.
• You might be pregnant and have sudden one-sided pain or bleeding.

See a doctor or physio soon if…

• The pain doesn’t change at all with movement or position.
• It wakes you at night and stays bad lying still.
• You notice changes in your urine or bowels.
• Muscle-type pain isn’t improving after two weeks.

Heat eases muscle pain. It can also dull pain from an organ and make you wait longer than you should. If you’re unsure which kind you have, get it checked first. More on this: when a heating pad helps back pain, and when it won’t.

Clinically reviewed by: Nigel Chua (Hand Occupational Therapist) & Louise Yow (Physiotherapist). This article is general information, not a diagnosis. electricheatpad.com sells electric heat pads.