Reclast (zoledronic acid) is a once-yearly infusion used to treat osteoporosis, and most of its side effects are mild and short-lived. But because the drug binds directly to bone and stays active for years, some effects can emerge or persist long after the infusion itself. The long-term concerns fall into a few distinct categories: bone complications, kidney strain, heart rhythm changes, eye inflammation, and low calcium levels.
The Acute Phase Reaction
This isn’t technically a long-term side effect, but it’s the most common complaint and worth understanding first. Many people experience flu-like symptoms within the first few days after their infusion: fever, muscle aches, headaches, and joint pain. These symptoms are most intense after the first dose and tend to be milder or absent with subsequent annual infusions. Taking acetaminophen before the infusion and continuing it for three days afterward can reduce the severity. If symptoms become severe, ibuprofen can be used as a backup.
Atypical Femur Fractures
The most concerning long-term risk with any bisphosphonate, including Reclast, is a rare type of thigh bone fracture. These aren’t ordinary breaks from a fall. They occur in the shaft of the femur, sometimes with little or no trauma, and are thought to result from the bone becoming overly suppressed in its normal remodeling process. Essentially, the drug does its job too well for too long, making bone more brittle in specific areas.
The risk increases sharply with duration of treatment. A large nationwide study found that the odds of atypical femur fracture roughly doubled for each additional year of bisphosphonate use. After four to five years of treatment, the odds ratio climbed to 116 compared to non-users. Notably, in that same study, none of the patients using zoledronic acid specifically developed an atypical fracture during the study period, which may reflect its less frequent dosing schedule (once yearly versus weekly oral pills). Still, the risk is considered a class effect of all bisphosphonates, and it’s the primary reason doctors recommend treatment breaks.
Osteonecrosis of the Jaw
Osteonecrosis of the jaw is a condition where bone tissue in the jaw dies and becomes exposed through the gums. It sounds alarming, and it is serious when it occurs, but in osteoporosis patients receiving Reclast, it is extremely rare. Across five clinical trials involving 5,903 patients, the incidence was less than one case per 14,200 patient-treatment-years. For context, that’s a fraction of a fraction of a percent.
The risk is substantially higher in cancer patients receiving much more frequent and higher doses of zoledronic acid, which is a different use case entirely. For osteoporosis patients getting one infusion per year, the risk is very low. Dental health does matter, though. Major dental procedures like tooth extractions or jaw surgery can increase vulnerability, so it’s worth keeping your dentist informed about your treatment.
Kidney Function Concerns
Reclast is cleared through the kidneys, and it can stress renal function. The FDA contraindicates it entirely in anyone with a creatinine clearance below 35 mL/min, which is a measure of how well your kidneys filter waste. There is no dose adjustment available for reduced kidney function; you either meet the threshold or you don’t receive the drug.
For people with adequate kidney function, the infusion still requires proper hydration. Acute kidney injury has been reported, particularly in patients who were dehydrated or already had borderline kidney function. Your doctor will check kidney function with a blood test before each infusion, and adequate fluid intake before and after the infusion helps protect against this risk.
Atrial Fibrillation
One of the more unexpected findings from clinical trials was a link between Reclast and serious heart rhythm disturbances. In the original HORIZON trial, serious atrial fibrillation (the kind requiring hospitalization) occurred in 1.3% of women receiving Reclast compared to 0.5% on placebo. Overall rates of atrial fibrillation were similar between groups, but the serious events were significantly more common with the drug. Most of these events occurred at least 30 days after the infusion, not immediately.
A larger observational study comparing Reclast to another osteoporosis drug (denosumab) confirmed a modestly higher risk. Among over 16,000 matched pairs of osteoporosis patients, the atrial fibrillation rate was 18.6 per 1,000 person-years with Reclast versus 14.9 with denosumab. The risk was even more pronounced in people who already had cardiovascular disease, where hospitalizations for atrial fibrillation were 60% higher with Reclast. In the extension study following patients for up to nine years, a small increase in cardiac arrhythmias persisted in those who continued treatment longest.
Low Calcium Levels
Reclast works by blocking the cells that break down bone. One consequence of shutting down that process is that less calcium flows from bone into the bloodstream, which can cause blood calcium to drop. Separately, increased bone-building activity after the infusion may pull even more calcium out of circulation. For most people with normal calcium and vitamin D levels, this isn’t a problem. But in people who are already deficient, it can cause symptoms like muscle cramps, tingling, and in severe cases, dangerous drops in calcium.
This is why calcium and vitamin D levels should be checked and corrected before each infusion, and why supplementation is recommended both before and after treatment.
Eye Inflammation
A lesser-known side effect is inflammatory eye conditions, including uveitis (inflammation inside the eye) and scleritis (inflammation of the white outer layer). Estimated incidence rates range from 0.046% to 1.1% for uveitis, with scleritis being even rarer. Symptoms include eye redness, intense pain, swelling of the eyelids, light sensitivity, and pain that can radiate toward the head. Because this side effect is uncommon, it often isn’t recognized quickly, which can delay treatment. If you develop significant eye pain or redness after a Reclast infusion, it’s worth flagging the infusion as a possible cause.
How Long Effects Last After Stopping
Unlike oral bisphosphonates or other osteoporosis drugs, zoledronic acid stays embedded in bone for years after you stop receiving infusions. This is actually an advantage in some ways. The HORIZON extension trial found that women who stopped after six annual infusions maintained their bone density benefits for up to three years without additional treatment. Bone turnover markers crept up slightly after stopping, but fracture rates didn’t significantly differ between those who continued and those who paused.
This residual effect is the basis for “drug holidays,” which current guidelines from the American Society for Bone and Mineral Research support. After meeting treatment goals, a pause in bisphosphonate therapy can be considered, and Reclast is the best candidate for this approach because its effects persist longest. The lingering activity may also mean that rare side effects like atypical fractures could theoretically still develop during a drug holiday, though the risk appears to decline once treatment stops. Periodic dosing beyond one year apart may be a viable strategy for long-term maintenance, though the optimal schedule is still being refined.
The overall risk profile of Reclast is favorable for most people with osteoporosis. The serious complications are genuinely rare, and the drug reduces fractures effectively. But because it accumulates in bone over years, understanding these long-term possibilities helps you weigh the benefits of continued treatment against the value of a well-timed break.