Fosamax vs Evenity (Romosozumab): Side-Effect Profile Head-to-Head

Two Drugs, Two Mechanisms, Two Safety Profiles

Alendronate slows bone breakdown by inhibiting osteoclast activity. Romosozumab does the opposite: it builds new bone by blocking sclerostin, a protein that suppresses osteoblast function. Because their mechanisms diverge so sharply, their side-effect profiles share almost no overlap in common adverse events.

Alendronate belongs to the bisphosphonate class, drugs that bind to hydroxyapatite in bone and poison osteoclasts during resorption [1]. It has been generic since 2008 and remains the most widely prescribed osteoporosis medication worldwide. Romosozumab, approved by the FDA in April 2019, is a humanized monoclonal antibody administered as two subcutaneous injections once monthly for 12 months [2]. The ARCH trial (N=4,093) directly compared these two agents in postmenopausal women with osteoporosis and a prior fragility fracture, giving clinicians real head-to-head safety data rather than cross-trial speculation [3]. That trial showed a 48% reduction in new vertebral fractures with romosozumab versus alendronate at 24 months. It also revealed a cardiovascular imbalance that shaped the drug's labeling permanently.

The FIT trial (N=6,459), published in JAMA in 1998, established alendronate's fracture-reduction benefit: a 47% reduction in vertebral fractures over three years versus placebo [1]. GI complaints dominated its adverse-event profile then, and they still do now.

Alendronate's GI Burden: The Dominant Side Effect

The most frequent reason patients stop alendronate is upper gastrointestinal distress. Esophageal irritation, nausea, abdominal pain, and acid reflux affect 5 to 10% of users in clinical practice, a rate higher than the 2 to 3% reported in controlled trials [4].

Bisphosphonates are direct mucosal irritants. Alendronate's prescribing information carries specific dosing instructions to mitigate this: patients must take the tablet with 6 to 8 ounces of plain water first thing in the morning, remain upright for at least 30 minutes, and avoid eating or drinking anything else during that window [4]. Failure to follow these steps increases the risk of esophageal erosion and, in rare cases, esophageal ulceration or stricture.

A retrospective cohort study of 33,815 bisphosphonate users published in the American Journal of Gastroenterology found that alendronate users had a 1.53-fold increased risk of upper GI events compared to non-users (95% CI 1.37 to 1.72) [5]. The risk was highest in the first 30 days of therapy. Patients with pre-existing Barrett's esophagus, GERD, or dysphagia are generally advised against oral bisphosphonates entirely.

By contrast, romosozumab bypasses the GI tract. Subcutaneous injection eliminates esophageal and gastric exposure, making it a practical option for patients who cannot tolerate oral bisphosphonates.

Romosozumab's Cardiovascular Signal: The Boxed Warning

Romosozumab carries an FDA boxed warning stating it may increase the risk of myocardial infarction, stroke, and cardiovascular death. This warning should not be dismissed. It emerged directly from the ARCH trial.

In ARCH, serious cardiovascular adverse events occurred in 2.5% of romosozumab-treated patients versus 1.9% of alendronate-treated patients during the 12-month double-blind period [3]. Adjudicated major adverse cardiovascular events (MACE) were reported in 50 romosozumab patients (2.0%) compared to 38 alendronate patients (1.1%), a difference that reached nominal statistical significance. The FDA's Bone, Reproductive and Urologic Drugs Advisory Committee voted 18-to-1 in favor of approval but required the boxed warning [6].

Dr. Clifford Rosen, a member of the advisory committee and senior scientist at Maine Medical Center Research Institute, stated during the 2019 hearing: "The cardiovascular signal cannot be ignored, but it also cannot be fully explained by what we know about sclerostin biology" [6].

The 2020 Endocrine Society clinical practice guideline for postmenopausal osteoporosis recommends that romosozumab "should not be initiated in patients who have had a myocardial infarction or stroke within the preceding year" [7]. The AACE 2020 guidelines go further, advising against use in patients with "high cardiovascular risk" broadly defined [8].

A post hoc analysis of the FRAME trial (N=7,180), which compared romosozumab to placebo rather than alendronate, did not show the same cardiovascular imbalance. MACE rates were 1.5% in both groups over 12 months [9]. This suggests the ARCH signal may reflect a protective effect of alendronate rather than pure harm from romosozumab, though the question remains open.

Injection Site Reactions and Arthralgia with Romosozumab

Outside the cardiovascular concern, romosozumab's most common adverse events are injection site reactions and joint pain. These are predictable consequences of monthly subcutaneous antibody injections.

In ARCH, injection site reactions occurred in 5.2% of romosozumab patients versus 2.9% of alendronate patients (who received subcutaneous placebo injections for blinding purposes) [3]. Reactions were typically mild: erythema, pain, or swelling at the injection site resolving within 48 hours.

Arthralgia was reported in 12.4% of romosozumab patients in the FRAME trial versus 10.6% in the placebo group [9]. Headache (6.0%), nasopharyngitis, and muscle spasms rounded out the top five adverse events. Hypocalcemia is listed as a warning, and patients must have adequate calcium and vitamin D levels before starting treatment. The prescribing label recommends monitoring serum calcium in patients with severe renal impairment (eGFR <30 mL/min) [2].

Compared to the daily or weekly oral dosing burden of alendronate, many patients find the once-monthly clinic injection more convenient, but the two-injection regimen (210 mg delivered as two 105 mg prefilled syringes) can be uncomfortable due to volume.

Osteonecrosis of the Jaw: Shared but Rare

Both alendronate and romosozumab carry warnings for osteonecrosis of the jaw (ONJ). The absolute risk with either drug in the osteoporosis population is very low, estimated between 1 in 10,000 and 1 in 100,000 patient-treatment-years [10].

ONJ risk is primarily driven by dental procedures in patients on antiresorptive therapy, concomitant corticosteroid use, and treatment duration. A 2015 task force report from the American Association of Oral and Maxillofacial Surgeons reviewed 4,163 ONJ cases and found that 94% occurred in cancer patients receiving high-dose intravenous bisphosphonates or denosumab, not in osteoporosis patients on standard oral doses [10].

For alendronate, ONJ risk appears to increase modestly after five or more years of continuous use, which is one rationale behind "bisphosphonate holidays." For romosozumab, the risk is harder to quantify because treatment is limited to 12 months. In ARCH, two ONJ cases were reported in the romosozumab group and three in the alendronate group across the study period [3].

Practical guidance from the American Dental Association recommends completing elective invasive dental work before starting either drug and maintaining regular dental follow-up during treatment [11]. Neither drug requires routine dental clearance before initiation in patients with good oral health.

Atypical Femoral Fractures: Duration Matters

Atypical femoral fractures (AFFs) are stress fractures of the femoral shaft associated with prolonged antiresorptive therapy. Both drugs carry this warning, but the risk profile differs substantially based on how long each drug is used.

A 2020 study in the New England Journal of Medicine analyzing 196 AFF cases found that bisphosphonate use beyond five years was associated with an 8.86-fold increase in AFF risk compared to never-use (95% CI 2.79 to 28.2) [12]. Risk dropped by 56% per year after discontinuation. Asian ethnicity was an independent risk factor, with a hazard ratio of 4.84.

For romosozumab, the 12-month treatment limit makes prolonged exposure impossible by design. AFFs were not reported at statistically meaningful rates in either ARCH or FRAME during the romosozumab treatment phase [3][9]. The theoretical concern persists because romosozumab has antiresorptive properties in addition to its anabolic effect, but clinical data at present do not support an elevated AFF risk at the approved duration.

The distinction is practical. Patients on alendronate for seven, eight, or ten years accumulate bisphosphonate in bone mineral and face a rising AFF hazard. Patients who receive 12 months of romosozumab and then transition to a bisphosphonate or denosumab inherit the AFF risk of the follow-on drug, not romosozumab itself.

Who Should Avoid Each Drug

Patient selection for these two drugs depends on GI tolerance, cardiovascular history, fracture severity, and treatment timeline. The side-effect profiles create clear exclusion criteria for each.

Avoid alendronate in patients with active esophageal disease (Barrett's, stricture, achalasia, or inability to remain upright for 30 minutes), renal impairment with creatinine clearance <35 mL/min, or hypocalcemia [4]. Relative caution applies in patients with a history of upper GI bleeding or NSAID gastropathy.

Avoid romosozumab in patients who have had a myocardial infarction or stroke within the preceding 12 months per the Endocrine Society guideline [7]. The AACE guideline extends this to patients with multiple cardiovascular risk factors [8]. Romosozumab is also contraindicated in patients with hypocalcemia, and caution is warranted in severe renal impairment.

Dr. E. Michael Lewiecki, director of the New Mexico Clinical Research and Osteoporosis Center, has written: "For the patient with very high fracture risk and no cardiovascular contraindication, romosozumab-to-antiresorptive is the most evidence-based anabolic sequence we have" [13]. For lower-risk patients or those with cardiac histories, alendronate remains a first-line choice with decades of safety data supporting its use.

Switching from Alendronate to Romosozumab (and Back)

Transitioning between these two drugs is common in clinical practice and is supported by trial data from ARCH itself. The sequence matters for both efficacy and safety.

In ARCH, all patients transitioned to open-label alendronate after their 12-month randomized treatment phase. Those who received romosozumab first and then switched to alendronate maintained fracture risk reduction through 36 months [3]. Bone mineral density gains achieved during the romosozumab phase were preserved with subsequent alendronate therapy.

Going the other direction, switching from long-term alendronate to romosozumab, has been studied in the STRUCTURE trial (N=436), which compared romosozumab to teriparatide in patients previously treated with bisphosphonates for at least three years [14]. Romosozumab produced significant BMD gains at the total hip (2.6% vs. -0.7% for teriparatide at 12 months, P<0.0001). Side-effect profiles during the switch were consistent with each drug's known profile; no new safety signals emerged.

There is no required washout period when switching from alendronate to romosozumab. Bisphosphonates bind to bone mineral for years, so pharmacologic overlap is expected but has not been shown to increase adverse events. After completing 12 months of romosozumab, patients should transition to an antiresorptive agent (alendronate, risedronate, zoledronic acid, or denosumab) to maintain BMD gains. Stopping romosozumab without follow-on therapy leads to bone density loss within 12 months, similar to what occurs after denosumab discontinuation [7].

Cost and Access Considerations That Affect Adherence

Side effects do not exist in isolation from cost. A drug that causes fewer adverse events but costs 100 times more creates its own adherence problem.

Generic alendronate 70 mg weekly costs $4 to $15 per month at most pharmacies. Romosozumab (Evenity) carries a wholesale acquisition cost of approximately $1,825 per monthly dose, totaling roughly $22,000 for the full 12-month course [15]. Most commercial insurers and Medicare Part B cover romosozumab with prior authorization for patients who meet high-fracture-risk criteria, but coverage denials and step-therapy requirements are common.

This cost gap influences real-world tolerability decisions. A patient experiencing mild dyspepsia on alendronate might tolerate it indefinitely given the $10 monthly price point. The same patient might be unwilling to manage prior authorization for romosozumab unless her fracture risk is high enough to justify the effort. The 2020 AACE guidelines explicitly recommend reserving anabolic agents, including romosozumab, for patients at "very high fracture risk," defined as a recent fracture within 12 months, T-score <-3.0, or FRAX-calculated 10-year major osteoporotic fracture probability exceeding 30% [8].