The most common side effect of osteoporosis infusions is a flu-like reaction that starts within 24 hours and typically resolves within one to three days. About half of patients experience this after their first infusion, but it becomes much less common with subsequent treatments. Beyond this short-term reaction, there are a few other side effects worth knowing about, depending on which type of infusion you receive.
The Acute Phase Reaction
The signature side effect of intravenous bisphosphonates (most commonly zoledronic acid, given once a year) is what doctors call an acute phase reaction. In a large clinical study, 51% of patients experienced at least one symptom within three days of their first infusion. The most common symptoms are fever, body aches, joint pain, headache, chills, and general fatigue. Symptoms typically begin within 16 hours of the infusion.
Fever tends to peak around 24 hours after the infusion and gradually returns to normal over the next 48 to 72 hours. Most people feel back to normal within three days. In some cases, joint pain or fatigue can linger for a week or two, and in rare instances, symptoms like generalized fatigue and joint pain have persisted for up to two months.
The good news: this reaction drops dramatically with repeat infusions. In one study, only 12% of patients had symptoms after their second annual infusion, compared to 51% the first time around. Your body essentially adjusts to the medication.
How to Reduce Flu-Like Symptoms
Taking 650 mg of acetaminophen (two regular-strength tablets) about 45 minutes before the infusion, then continuing that dose every six hours for three days afterward, can meaningfully reduce post-infusion symptoms. This protocol was tested in a randomized trial of nearly 800 postmenopausal women and is now a standard recommendation. Staying well-hydrated before and after the infusion also helps. While no specific water intake has been formally established, research suggests that receiving at least 500 mL of intravenous fluids around the time of infusion helps protect kidney function.
Kidney Effects
Zoledronic acid is processed through the kidneys, so it can temporarily affect kidney function. This is why your care team will check your kidney function with a blood test before each infusion, and why adequate hydration matters. Patients with significantly impaired kidney function are generally not candidates for this type of infusion. If your kidneys are healthy, the risk is low, especially with proper hydration.
Low Calcium Levels
Both bisphosphonate infusions and denosumab (a different type of osteoporosis injection given every six months) can cause blood calcium levels to drop. For denosumab, hypocalcemia is the most frequently reported adverse event in safety databases, though the actual incidence in osteoporosis patients is low. One hospital-based study found that 14% of denosumab patients developed low calcium within six months, while broader estimates put the rate below 0.05%. The discrepancy likely reflects differences in monitoring and patient populations. Either way, your doctor will check your calcium and vitamin D levels before treatment and may have you take supplements to keep levels in a safe range.
Rare but Serious Complications
Jawbone Damage
Osteonecrosis of the jaw, where a section of jawbone loses its blood supply and begins to break down, is the most well-known rare complication of osteoporosis medications. In osteoporosis patients (as opposed to cancer patients receiving much higher doses), the estimated incidence is between 1 in 10,000 and 1 in 100,000. Dental work like tooth extractions or implants increases the risk, which is why your doctor may recommend completing any major dental procedures before starting treatment.
Unusual Thigh Fractures
Atypical femur fractures, stress fractures in the thigh bone that occur with minimal trauma, are another rare concern with long-term bisphosphonate use. The risk is very low in the first few years of treatment (about 1.8 per 100,000 person-years after two years), but it increases with duration. After eight to ten years of continuous treatment, the rate rises to roughly 113 per 100,000 person-years. This is one reason doctors sometimes recommend a “drug holiday” after several years of bisphosphonate therapy. New or unusual thigh pain during treatment is worth reporting to your doctor.
Side Effects Specific to Denosumab
Denosumab is technically a subcutaneous injection rather than an IV infusion, but many patients searching for osteoporosis infusion side effects are receiving or considering it. Unlike bisphosphonates, denosumab does not cause the acute flu-like reaction. Its side effect profile instead includes skin reactions, eczema, and a slightly higher rate of skin infections and urinary tract infections.
The most important thing to know about denosumab is what happens when you stop it. Discontinuing the medication can trigger a rebound effect where bone density drops rapidly, sometimes below pre-treatment levels, and the risk of vertebral fractures increases significantly. This means that once you start denosumab, you need a clear plan for either continuing it or transitioning to another medication. Stopping abruptly is not recommended.
Cardiovascular Concerns With Romosozumab
Romosozumab is a newer monthly injection (not an IV infusion) for osteoporosis that works differently from bisphosphonates or denosumab by actively building new bone. It carries a boxed warning from the FDA cautioning against use in patients who have had a heart attack or stroke within the past year, due to a potential increase in cardiovascular events seen in clinical trials. Treatment is limited to 12 months, after which patients typically transition to another osteoporosis medication.
What to Expect on Infusion Day
Before your first zoledronic acid infusion, you’ll have blood work to check kidney function, calcium, and vitamin D levels. The infusion itself takes at least 15 minutes and is given through a standard IV line. You’ll likely be encouraged to drink plenty of water before arriving. Most infusion centers will have you stay briefly afterward to make sure you’re feeling fine.
Plan for the possibility of feeling achy or feverish that evening and the next day. Many patients describe it as feeling like a mild flu. Having acetaminophen on hand, staying hydrated, and keeping your schedule light for 48 hours after the infusion is practical advice. If you sailed through your first infusion without symptoms, your odds of reacting to the next one are even lower.