Bisphosphonates: How They Work, Uses, and Side Effects

Bisphosphonates are a class of drugs that slow bone loss by interfering with the cells that break down bone. They have been the backbone of osteoporosis treatment for decades, and meta-analyses consistently show they cut fracture risk by roughly a third to a half depending on the fracture site. Beyond osteoporosis, they play a significant role in cancer care, rare bone diseases in children, and steroid-induced bone loss. Their long residence time in bone tissue, sometimes measured in years, makes them uniquely effective but also raises questions about how long to stay on them and what happens when you stop.

How Bisphosphonates Work

Your skeleton is constantly being remodeled. Specialized cells called osteoclasts dissolve old bone, while osteoblasts build new bone in its place. In osteoporosis, osteoclasts outpace osteoblasts, and bone gradually thins. Bisphosphonates work by targeting osteoclasts and either killing them or hobbling their activity. They bind tightly to bone mineral, especially at sites where bone turnover is highest, and get taken up by osteoclasts when those cells start dissolving the bone surface.

There are two broad types, and they disable osteoclasts in different ways. The older, simpler bisphosphonates like clodronate get converted inside the osteoclast into toxic versions of the cell’s own energy molecule (ATP), which triggers the cell to self-destruct.¹ The newer nitrogen-containing bisphosphonates, which include alendronate, risedronate, and zoledronic acid, work differently. They block an enzyme called farnesyl pyrophosphate synthase inside the osteoclast, disrupting the chemical signals the cell needs to function and survive.² The nitrogen-containing versions are more potent, which is why they dominate modern prescribing.

One unusual feature is how long bisphosphonates stick around. Once deposited in bone, they stay there until that particular patch of bone is eventually resorbed. The half-life in bone tissue ranges from roughly one to ten years depending on how fast a person’s skeleton turns over.³ This is why bisphosphonates keep working even after you stop taking them, and it is central to the concept of “drug holidays” discussed further on.

How Much They Reduce Fracture Risk

The core reason anyone takes a bisphosphonate is to avoid breaking a bone, and the evidence here is solid. A meta-analysis pooling data from multiple randomized trials found that bisphosphonate use lowered overall osteoporotic fracture risk by about 38%, with vertebral (spinal) fractures dropping by roughly 45% and non-vertebral fractures by about 27%.⁴ A separate meta-analysis focusing on postmenopausal women with low bone density found an even larger reduction for vertebral fractures, around 50%.⁵

Spinal fractures respond better to treatment than hip fractures, which is a pattern seen across most osteoporosis drugs. This likely reflects the fact that the spine has a higher proportion of the type of bone (trabecular bone) that turns over quickly, giving bisphosphonates more tissue to bind to and protect. Hip fractures involve more dense cortical bone, where turnover is slower and the drugs accumulate less aggressively. Even so, the reduction in hip and other non-spinal fractures is meaningful and consistent across trials.

Taking Oral Bisphosphonates

If you have ever been prescribed alendronate or risedronate, you know the dosing routine is unusually demanding. The tablets must be swallowed on a completely empty stomach with a full glass of plain water, and you cannot eat, drink anything else, or lie down for at least 30 minutes afterward.⁶ Some formulations ask you to wait a full two hours before eating. These instructions exist for two reasons: the drugs are absorbed very poorly (typically less than 1% of the dose enters the bloodstream), so any food or drink drastically cuts absorption further, and the pills can irritate the esophagus if they linger there.

When people do not follow the instructions, gastrointestinal side effects are common. These can include heartburn, nausea, and in more severe cases, inflammation or ulceration of the esophagus.⁷ The inconvenience of this routine is one of the main reasons people stop taking bisphosphonates prematurely. Weekly dosing (instead of daily) was developed partly to reduce this burden, and intravenous options like zoledronic acid, given once a year, eliminate the gastrointestinal issue entirely.

The Flu-Like Reaction After Infusion

Intravenous zoledronic acid avoids stomach problems but introduces a different short-term side effect: an acute-phase reaction that feels like a mild flu. In the large HORIZON trial, about 42% of women receiving their first zoledronic acid infusion experienced this reaction within three days, compared to about 12% receiving a placebo.⁸ Symptoms include fever, muscle aches, headache, and fatigue. They typically resolve within a day or two and tend to be milder or absent with subsequent annual infusions.

Interestingly, the same HORIZON data showed that women who had this reaction actually fared better over the long run. Those who experienced the flu-like symptoms after their first infusion had a lower rate of vertebral fractures over the following three years, with fracture incidence roughly half that of women on zoledronic acid who did not have the reaction.⁹ The reaction may signal a more robust immune and skeletal response to the drug. So while the short-term discomfort is unpleasant, it is not a sign that something has gone wrong.

Rare but Serious Risks

Two uncommon complications get the most attention: osteonecrosis of the jaw and atypical femur fractures. Both are genuinely serious, but both are also genuinely rare, and understanding who is most at risk matters more than simply knowing they exist.

Jaw Osteonecrosis

Osteonecrosis of the jaw (ONJ) is a condition where a patch of jawbone becomes exposed and fails to heal. It is most strongly linked to the high-dose intravenous bisphosphonates used in cancer care, not the lower doses prescribed for osteoporosis. A case-control study found that intravenous bisphosphonate use carried an odds ratio for ONJ of about 300 compared to non-use, while oral bisphosphonates had an odds ratio of about 12.¹⁰ That sounds alarming in relative terms, but the baseline risk is extremely low, so even a large multiplier produces a small absolute number. The vast majority of cases occur in cancer patients receiving monthly intravenous infusions of zoledronic acid or pamidronate.¹¹

Dental surgery, particularly tooth extraction, is a major triggering event. Roughly 60% of reported cases followed some kind of oral surgery. This is why dentists ask about bisphosphonate use before procedures, and why completing any major dental work before starting bisphosphonate therapy is standard advice. For someone taking a weekly oral bisphosphonate for osteoporosis who has not had dental extractions or jaw trauma, the risk of ONJ is very small.

Atypical Femur Fractures

Atypical femur fractures are stress fractures that occur along the shaft of the thighbone rather than at the hip joint, and they look different from ordinary osteoporotic fractures on an X-ray. The risk rises with longer duration of bisphosphonate use. A large study found that compared to use of less than three months, three to five years of use raised the hazard ratio to about 9, while eight or more years raised it to about 44.¹² Again, these are relative increases from a very low baseline, so the absolute risk remains small. And reassuringly, the same study showed that the risk dropped rapidly after discontinuation.

The underlying mechanism is thought to relate to the very thing that makes bisphosphonates work. By suppressing bone remodeling so effectively, the drugs can prevent the normal repair of tiny stress cracks (microdamage) that accumulate with everyday use. Over many years, those unrepaired microcracks can coalesce in high-stress areas like the outer wall of the femur shaft.¹³ This is the central tension in bisphosphonate therapy: the same mechanism that prevents fragility fractures can, over a long enough period, create a different kind of fracture risk.

Drug Holidays

Because of the long-term risks described above and because bisphosphonates continue to protect bone even after you stop taking them, most guidelines now recommend a planned break after several years of treatment. For patients at moderate risk, a drug holiday of two to three years is standard after about five years of oral therapy or three years of intravenous zoledronic acid.¹⁴

A meta-analysis of studies on drug holidays found that women who stopped bisphosphonates after three to five years had no significantly higher risk of hip fractures or other clinical fractures compared to women who continued without interruption. The researchers noted that women with hip bone density that remained reasonably good at the time of discontinuation were the best candidates for a break, while those with persistently low hip density might benefit from continued treatment.¹⁵ During a holiday, doctors can monitor bone turnover markers in the blood. A rise in these markers above a meaningful threshold signals that the drug’s residual effect is wearing off and it may be time to restart treatment.¹⁶

Use in Cancer Patients

Outside of osteoporosis, bisphosphonates play a major role in managing bone complications from cancer. Many tumors, particularly breast, prostate, and kidney cancers, spread to bone and cause painful fractures, spinal cord compression, and other skeletal damage. Intravenous zoledronic acid has been shown to reduce these events in patients with bone metastases from prostate and kidney cancers, delaying the time to the first complication and lowering the overall proportion of patients who experienced one.¹⁷

In kidney cancer specifically, the data are striking. In a subset of 74 patients, zoledronic acid cut the proportion experiencing a skeletal event from about three-quarters in the placebo group to about a third, and reduced the overall risk of a skeletal event by 61%.¹⁸ The doses used in cancer care are higher and given more frequently than in osteoporosis, which is why the risk of jaw osteonecrosis is so much greater in this population.

Steroid-Induced Bone Loss and Childhood Bone Disorders

People who take corticosteroids like prednisone for conditions such as autoimmune diseases or asthma are at heightened risk of bone loss, and bisphosphonates are the first-line defense. Oral bisphosphonates are recommended as first-choice treatment for steroid-induced osteoporosis largely because they are effective and inexpensive.¹⁹ In men on glucocorticoids, a trial comparing intravenous zoledronic acid with oral risedronate found that zoledronic acid produced larger gains in spine and hip bone density, though both drugs prevented further bone loss.²⁰

Children with osteogenesis imperfecta, a genetic condition that makes bones extremely fragile, are another group who benefit. A Cochrane review of trials found that bisphosphonates consistently increased spine bone density in these children.²¹ A randomized trial of oral risedronate in children with the condition showed increased bone density and a reduced risk of both first and recurrent fractures.²² A broader systematic review confirmed the pattern: most studies found that bisphosphonate treatment led to higher spine density and fewer fractures in children with this disorder.²³

How Bisphosphonates Fit With Newer Drugs

Bisphosphonates are anti-resorptive, meaning they slow bone breakdown. But a newer approach uses anabolic (bone-building) drugs like teriparatide and romosozumab that stimulate new bone formation. For patients at the highest risk of fractures, starting with an anabolic drug and then switching to a bisphosphonate produces better outcomes than starting with a bisphosphonate alone.²⁴ The anabolic drugs build bone mass rapidly, and the bisphosphonate locks those gains in place afterward. The bone-building effect of anabolic agents is self-limiting, so following them with an anti-resorptive is considered essential.²⁵

Denosumab, another anti-resorptive that works through a different pathway, presents its own set of issues when it comes time to stop. Discontinuing denosumab causes a rebound surge in bone breakdown, rapid bone density loss, and in some cases, spontaneous vertebral fractures.²⁶ Bisphosphonates are the go-to transition strategy to cushion this rebound, though how well they work depends on how long the patient was on denosumab. A systematic review and meta-analysis supports using bisphosphonates after denosumab discontinuation, aligning with current guidelines, but acknowledges that the best approach for patients on long-term denosumab remains unsettled.²⁷ This makes bisphosphonates something of an anchor drug in osteoporosis management: they are often where treatment starts, but they are also frequently where treatment returns after a course of something else.

Kidney Safety

Bisphosphonates are cleared from the body through the kidneys, which has raised ongoing concern about use in people with impaired kidney function. In animal studies, high doses given rapidly have caused kidney damage ranging from scarring of the filtering units to acute cell death in the kidney tubules. However, in the doses used for osteoporosis in people, oral bisphosphonates have never been shown to harm the kidneys, even in analyses that included patients with estimated kidney filtration rates as low as 15 mL per minute, which represents severe impairment.²⁸ Intravenous bisphosphonates require more caution: zoledronic acid is generally not recommended when kidney function falls below a certain threshold, and infusions must be given slowly to avoid kidney stress. Most prescribers check kidney function before initiating treatment and periodically during therapy.

Effects Beyond Bone

Because bisphosphonates interfere with calcification in general, researchers have explored whether they might also slow the hardening of arteries, a process that contributes to cardiovascular disease. In animal experiments, alendronate and ibandronate completely prevented calcification of arteries and heart valves at doses similar to those that suppress bone loss.²⁹ Observational and trial data in humans suggest a possible reduction in cardiovascular events, but the evidence is not strong enough to prescribe bisphosphonates for heart protection.³⁰ It remains an area of active research and a potential bonus for patients already taking these drugs for their bones.

Other non-skeletal effects are also being investigated. The same enzyme pathway that bisphosphonates block in osteoclasts exists in other cell types, and researchers have explored whether this might translate to anticancer, anti-inflammatory, or anti-parasitic activity.³¹ These are largely laboratory and early-stage findings. For now, the practical clinical value of bisphosphonates remains centered on bone, but the story of what these drugs do beyond the skeleton is still being written.