Muscle aches have dozens of possible causes, ranging from yesterday’s workout to an underlying illness your body is fighting off. The most common triggers fall into a few broad categories: physical exertion, infections, nutrient deficiencies, medications, and chronic inflammatory conditions. Understanding which category fits your situation helps you figure out whether your aches will resolve on their own or need medical attention.
Exercise and Physical Overexertion
The soreness you feel a day or two after a hard workout is called delayed onset muscle soreness, or DOMS. It typically peaks around 48 hours after exercise and fades within a few days. Despite what many people still believe, this soreness is not caused by lactic acid buildup. Lactic acid clears from your muscles within an hour or so of stopping exercise.
What actually happens is structural. When muscles work against heavy loads, especially during movements where the muscle lengthens under tension (like lowering a heavy weight, running downhill, or the lowering phase of a squat), the force creates microscopic tears in muscle fibers. This damage triggers a chain reaction: calcium floods into the injured cells, immune cells called macrophages move in to clean up the debris, and the byproducts of that cleanup process irritate nearby nerve endings. That irritation is what you feel as soreness and stiffness. It’s a normal part of how muscles adapt and grow stronger, but it can be surprisingly intense after unfamiliar exercises or a big jump in training volume.
Viral and Bacterial Infections
That all-over body ache you get with the flu, COVID-19, or even a bad cold isn’t the virus directly attacking your muscles. It’s your immune system’s doing. When your body detects an invading pathogen, it releases a flood of inflammatory signaling molecules called cytokines. One of the most important players is interleukin-6 (IL-6), which triggers the production of pain-sensitizing chemicals in muscle tissue. The more aggressively your immune system responds, the worse the aching tends to be.
This explains why severe infections often cause more intense muscle pain. In serious COVID-19 cases, for example, researchers found dramatically elevated levels of IL-6 along with several other inflammatory signals, including tumor necrosis factor. The rapid replication of the virus drives the immune system into overdrive, and the resulting “cytokine storm” is strongly linked to the widespread muscle pain many patients report. The aches usually resolve as the infection clears and cytokine levels drop back to normal, though some people experience lingering muscle pain for weeks or months after certain infections.
Nutrient Deficiencies
Low levels of certain vitamins and minerals can produce muscle aches that feel vague and hard to pin down. Vitamin D deficiency is one of the most well-studied connections. A large meta-analysis found that people with muscle pain had vitamin D levels roughly 9 nmol/L lower on average than people without pain, and similar patterns showed up in people with chronic widespread pain. Vitamin D plays a role in muscle function and calcium absorption, so when levels drop too low, muscles can ache, feel weak, or cramp more easily.
Potassium and magnesium deficiencies also cause muscle symptoms. Normal potassium levels fall between 3.5 and 5.3 mmol/L, and when they drop significantly below that range, you can experience fatigue, weakness, cramps, and a general aching quality in your muscles. Magnesium deficiency often travels with low potassium, since magnesium helps regulate potassium balance. Together, these deficiencies can produce persistent muscle discomfort, tingling, and cramping that doesn’t respond to rest or stretching. In severe cases, the neuromuscular symptoms can progress to spasms or even temporary paralysis. These deficiencies are more common in people who take certain diuretics, have digestive conditions that impair absorption, or eat very restricted diets.
Medication Side Effects
Statins, the cholesterol-lowering drugs taken by tens of millions of people worldwide, are one of the most recognized medication-related causes of muscle aches. Estimates of how often this happens vary widely. In controlled clinical trials, about 5% of people on statins report muscle symptoms. In real-world studies where patients self-report, the number climbs to 10 to 20%. A large meta-analysis covering over 4 million patients settled on an overall rate of about 9%.
The mechanism is complex but centers on energy production inside muscle cells. Statins block a metabolic pathway that produces cholesterol, but that same pathway also produces coenzyme Q10, a molecule your mitochondria need to generate energy. With less coenzyme Q10 available, muscle cells produce less energy and generate more damaging waste products called reactive oxygen species. This oxidative stress injures muscle fibers over time. Statins also disrupt calcium regulation within muscles, which can cause cramps and spasms, and they activate genes involved in breaking down muscle protein, which may contribute to gradual muscle weakening.
Beyond statins, other medications that commonly cause muscle aches include certain blood pressure drugs, corticosteroids used long-term, and some antibiotics. If muscle pain starts shortly after beginning a new medication, the timing itself is an important clue.
Chronic Inflammatory Conditions
When muscle aches persist for months without a clear cause, two conditions worth knowing about are fibromyalgia and polymyalgia rheumatica.
Fibromyalgia causes widespread pain that lasts at least three months and affects multiple body regions. Under the current diagnostic criteria, pain must be present in at least four of five body regions (both upper and lower body, both sides, plus the spine area). The pain is often accompanied by fatigue, sleep problems, and cognitive difficulties sometimes called “brain fog.” Fibromyalgia doesn’t show up on blood tests or imaging, which is part of why it takes an average of several years to diagnose. It’s thought to involve the way the brain and spinal cord process pain signals, essentially turning up the volume on normal sensations.
Polymyalgia rheumatica is a different picture entirely. It almost exclusively affects people over 50 and causes severe stiffness and aching in the shoulders, hips, and neck, particularly in the morning. Unlike fibromyalgia, it does show up on blood work. Doctors look for elevated inflammatory markers: an erythrocyte sedimentation rate (ESR) of 40 mm/h or higher and a C-reactive protein (CRP) above 6 mg/L are classic findings. Morning stiffness lasting more than an hour is a hallmark. Polymyalgia rheumatica responds dramatically well to treatment, with most people feeling significantly better within days of starting therapy.
Dehydration and Electrolyte Imbalances
Your muscles are roughly 75% water, and even mild dehydration changes how they function. When you’re low on fluids, blood flow to muscles decreases, waste products build up more slowly, and electrolyte concentrations shift. The result is often a dull aching sensation, increased tendency to cramp, and slower recovery after activity. This is especially common in hot weather, after prolonged sweating, during illness with vomiting or diarrhea, and in people who simply don’t drink enough water throughout the day.
Stress and Sleep Deprivation
Chronic stress keeps your muscles in a state of low-level tension, particularly in the neck, shoulders, and upper back. Over time, this sustained contraction restricts blood flow to the affected muscles and creates a cycle of tension, reduced oxygen delivery, and pain. Poor sleep compounds the problem. During deep sleep, your body releases growth hormone and carries out most of its tissue repair. When sleep is consistently disrupted or too short, muscle recovery slows and your pain threshold drops, meaning normal sensations register as more painful than they would otherwise.
Red Flags That Need Prompt Attention
Most muscle aches are temporary and harmless. But certain patterns signal something more serious. Rhabdomyolysis, a condition where damaged muscle fibers break down and release their contents into the bloodstream, requires urgent care. Warning signs include severe muscle pain and weakness, dark tea-colored urine, swelling in the affected area, fever, confusion, or nausea. The breakdown products can damage the kidneys, and the risk of kidney injury rises significantly when muscle enzyme levels exceed about 5,000 U/L, roughly 10 times the normal upper limit.
Other concerning patterns include muscle aches that worsen steadily over weeks, pain concentrated around the hips and shoulders with difficulty raising your arms or climbing stairs (which can suggest inflammatory muscle disease), aches accompanied by an unexplained rash, or muscle pain with significant unintentional weight loss. Any of these combinations warrants a closer look with blood work and a thorough physical exam.