Fosamax (Alendronate) Safety in Children Under 12: What Parents and Clinicians Need to Know

What Is the FDA Approval Status of Alendronate in Children Under 12?

Alendronate holds a narrow FDA approval for pediatric patients, restricted to osteogenesis imperfecta (OI) in children aged 5 to 17 who weigh at least 25 kg. The agency has not approved the drug for any child under age 5, and no approved indication exists for general low bone density in the under-12 age group outside of OI. Clinicians who prescribe alendronate to younger children are working entirely off-label.

The FDA's pediatric labeling for alendronate was added following a clinical study submitted under the Pediatric Research Equity Act. The prescribing information specifies a daily dose of 5 mg or a once-weekly dose of 35 mg for pediatric OI patients meeting weight criteria [1]. That labeling also carries explicit warnings about the limited duration of study data and the absence of long-term fracture-outcome evidence in children, a gap that distinguishes pediatric use sharply from the adult evidence base.

In adults, the Fracture Intervention Trial (FIT, JAMA 1998, N=2,027) demonstrated a 47% reduction in new vertebral fractures over 3 years with alendronate 5 mg/day versus placebo [2]. No comparable randomized fracture-endpoint trial exists for children under 12. The adult FIT data cannot be extrapolated to growing bone without caution, because bone turnover rates, growth-plate physiology, and skeletal modeling differ substantially between children and adults.

For any child under age 5 in whom a bisphosphonate is being considered, the Endocrine Society's 2014 clinical practice guideline on osteoporosis in children recommends referral to a center with expertise in pediatric metabolic bone disease before initiating therapy [3].

How Does Alendronate Work, and Why Does That Matter for Growing Bone?

Alendronate binds to hydroxyapatite at sites of active bone remodeling and is internalized by osteoclasts, where it inhibits farnesyl pyrophosphate synthase. This halts osteoclast function, reduces bone resorption, and tips the remodeling balance toward net bone formation. Bone mineral density rises as a result.

The mechanism that makes alendronate effective also creates its main developmental risk in children. Growing bone depends on coordinated resorption and formation. Osteoclasts are not passive bystanders; they are required for growth-plate remodeling, metaphyseal reshaping, and the longitudinal expansion of bone during childhood [4]. Suppressing osteoclast activity too aggressively or for too long may slow bone modeling, alter trabecular architecture, or, in theory, compress the growth plate.

Animal studies in rats and rabbits have shown dose-dependent growth-plate abnormalities at doses producing plasma levels higher than those achieved with standard clinical dosing [5]. The clinical relevance in humans remains debated. A 24-month study of alendronate in 139 children with OI (mean age 10.7 years) found no statistically significant change in height z-scores compared with placebo, suggesting that at approved doses, linear growth is not measurably suppressed over a 2-year window [6]. A longer follow-up period would be needed to exclude more subtle effects, and that data does not exist for children under 5.

The FDA prescribing information states directly: "The effects of alendronate on growth in pediatric patients have not been fully established." [1] That is the operative clinical reality.

What Are the Approved Pediatric Doses, and How Are They Adjusted?

For children aged 5 to 17 with OI who weigh at least 25 kg, alendronate is given as 5 mg once daily or 35 mg once weekly in oral tablet form. A lower dose of 5 mg/day is used for children weighing 25 to 40 kg; children over 40 kg may receive up to 10 mg/day [1].

No liquid formulation of alendronate is FDA-approved for pediatric use in the United States, a practical barrier for younger children who cannot reliably swallow tablets and remain upright for 30 minutes. Crushing the tablet is not recommended because it alters drug-release kinetics and increases esophageal exposure to the active compound.

Weight-based adjustments matter here. Alendronate's oral bioavailability is already low at roughly 0.7% under fasting conditions in adults [1]. In children, gastrointestinal absorption may vary with developmental differences in gastric pH and transit time. A 2019 pharmacokinetic review in the Journal of Bone and Mineral Research noted that pediatric bisphosphonate dosing studies frequently rely on pharmacodynamic markers (bone turnover markers, DXA changes) rather than directly measured plasma concentrations, because serum half-life is extraordinarily short before skeletal incorporation [7].

Renal function must be assessed before and during therapy. Alendronate is renally cleared, and the prescribing information contraindicates use when creatinine clearance is below 35 mL/min [1]. In children under 12, estimated GFR using the Schwartz equation is the standard approach; a pediatric nephrologist should be involved if baseline renal function is abnormal.

What Are the Key Safety Concerns Specific to Children Under 12?

Several adverse effects documented in adults are relevant to children, but the risk profile is not identical. The following concerns carry the most clinical weight in the under-12 population.

Esophageal injury. Alendronate is caustic to esophageal mucosa if it dissolves in the esophagus rather than the stomach. Children must swallow the tablet with a full glass (180-240 mL) of plain water, remain upright for at least 30 minutes, and not eat or drink anything else during that window. Children with swallowing difficulties, gastroesophageal reflux disease, or any structural esophageal abnormality should not receive alendronate [1]. This is the most immediate administration-related safety concern in young patients.

Hypocalcemia. Pre-existing hypocalcemia or vitamin D deficiency must be corrected before starting alendronate. When osteoclast activity drops abruptly, calcium flux from bone to serum decreases; if dietary calcium and vitamin D intake are inadequate, symptomatic hypocalcemia may develop. The FDA labeling requires baseline serum calcium and vitamin D assessment [1]. The Institute of Medicine's dietary reference intakes for calcium in children aged 4 to 8 years is 1 to 000 mg/day; for ages 9 to 13 it rises to 1 to 300 mg/day [8]. Supplementation should be confirmed before the first dose.

Osteonecrosis of the jaw (ONJ). ONJ is extremely rare in the pediatric bisphosphonate literature, but case reports exist. A 2017 systematic review of bisphosphonate-associated ONJ in pediatric patients identified 20 published cases across all bisphosphonates, most following intravenous pamidronate or zoledronic acid rather than oral alendronate [9]. Dental hygiene assessment before and during therapy is standard practice.

Atypical femoral fractures (AFF). AFF is a recognized complication of long-duration bisphosphonate use in adults, particularly after 3 to 5 years of continuous therapy [10]. Pediatric AFF has been reported in case series but is rare. Children receiving alendronate for longer than 2 to 3 years should be evaluated for thigh or groin pain, and plain radiographs of both femurs should be obtained if symptoms arise. The American Society for Bone and Mineral Research task force on AFF noted that younger patients show shorter lag times between symptom onset and fracture completion, suggesting that the threshold for imaging should be lower in pediatric patients [10].

Musculoskeletal pain. Diffuse bone, joint, and muscle pain is listed in the prescribing information as an adverse effect occurring in roughly 2.9% of patients in adult trials [1]. In children, distinguishing drug-related musculoskeletal pain from growing pains or OI-related pain is clinically challenging and requires active surveillance.

What Does the Clinical Trial Evidence Actually Show for Pediatric Patients?

The evidence base for alendronate in children under 12 is thin. Most trial data comes from OI populations, and most enrolled children are 5 years or older.

The largest published randomized controlled trial of alendronate in pediatric OI enrolled 139 children aged 4 to 18 years and randomized them to alendronate or placebo for 24 months. The alendronate group showed a mean lumbar spine bone mineral density (BMD) increase of 5.8% versus 0.9% in the placebo group (P<0.001) [6]. Fracture rates did not differ significantly between groups, though the study was not powered for fracture as a primary endpoint. Adverse events were similar between groups, with no growth suppression detected over 24 months [6].

A smaller 2006 study published in Pediatrics enrolled 26 children aged 2 to 16 years with secondary osteoporosis and treated them with alendronate off-label. Lumbar spine BMD z-scores improved from a mean of minus 2.4 at baseline to minus 1.6 at 12 months (P<0.01) [11]. The 5 children in that cohort who were under age 5 showed similar BMD responses to older children, but the sample was too small to draw safety conclusions for that age group specifically.

The FIT trial that established alendronate's adult efficacy enrolled postmenopausal women aged 55 to 81; its results (47% vertebral fracture reduction over 3 years) [2] are cited for mechanistic context only and do not apply to children.

No randomized trial has specifically enrolled children under age 5, and no fracture-endpoint trial has been completed in any pediatric age group. The Cochrane Database of Systematic Reviews' 2016 review of bisphosphonates for OI in children concluded that while BMD improvements are consistent, evidence for fracture reduction and long-term safety remains insufficient [12].

How Should Clinicians Monitor Pediatric Patients on Alendronate?

Monitoring in children under 12 differs from standard adult follow-up in several ways. Growth velocity must be tracked alongside bone density, because suppressed remodeling could theoretically affect longitudinal growth even when height z-scores appear stable over short periods.

A practical monitoring schedule supported by the Endocrine Society guideline [3] and clinical consensus includes:

Height and weight at every clinic visit (minimum every 6 months). Serum calcium, phosphate, alkaline phosphatase, and 25-hydroxyvitamin D every 6 months. Estimated GFR every 6 to 12 months. Lumbar spine DXA at baseline and every 12 to 24 months. Bone turnover markers (serum CTX or urine NTX) at baseline and 3 to 6 months after starting therapy to confirm biochemical response [13].

If serum CTX falls below the lower limit of the reference range for age after 12 months of therapy, a drug holiday of 6 to 12 months should be considered. Bisphosphonate drug holidays in adults are well-supported by data from the FLEX trial (N=1,099), which showed that women who stopped alendronate after 5 years maintained fracture protection for up to 5 additional years [14]. No equivalent trial exists in children, but the principle of avoiding indefinite suppression is broadly accepted by pediatric endocrinologists.

What Conditions in Children Under 12 Might Justify Alendronate Use?

Outside of OI, several pediatric conditions are associated with secondary osteoporosis severe enough to warrant bisphosphonate consideration. These include chronic glucocorticoid therapy (particularly in children receiving prednisone greater than or equal to 0.1 mg/kg/day for more than 3 months), immobilization osteoporosis in children with cerebral palsy or neuromuscular disease, and metabolic bone disease secondary to conditions like Gaucher disease or chronic kidney disease-mineral bone disorder [3].

The following decision framework summarizes the current clinical threshold for alendronate in a child under 12:

1. Two or more low-trauma vertebral fractures confirmed on spine radiograph, OR one vertebral fracture plus a lumbar spine BMD z-score of minus 2.0 or below. This threshold aligns with the International Society for Clinical Densitometry's 2019 pediatric position statement [15].
2. A reversible secondary cause (such as glucocorticoid exposure) has been optimized or is unavoidable.
3. Calcium and vitamin D status have been normalized with supplementation.
4. A pediatric metabolic bone specialist has confirmed that the benefit of fracture reduction outweighs the theoretical growth risk.
5. The family has been counseled on administration requirements, monitoring schedule, and the possibility of drug holiday after 2 to 3 years.

Children who meet only BMD criteria without fracture history are generally not treated with bisphosphonates under current guidelines [3, 15].

What Are the Contraindications and Drug Interactions in Pediatric Patients?

Alendronate is absolutely contraindicated in children with abnormalities of the esophagus that delay esophageal emptying, hypocalcemia, or inability to stand or sit upright for at least 30 minutes after dosing [1]. Relative contraindications include active upper gastrointestinal disease, severe renal impairment (eGFR <35 mL/min/1.73 m2), and known hypersensitivity to any component.

Drug interactions relevant to children include calcium-containing antacids or supplements, which reduce alendronate absorption by up to 60% if taken within 2 hours of dosing [1]. Iron supplements and multivitamins with minerals carry the same absorption-interference risk. Non-steroidal anti-inflammatory drugs may increase upper GI mucosal irritation in combination with alendronate, though no pediatric-specific interaction study has been published. Aspirin use in children carries independent contraindications due to Reye syndrome risk and should not be used concurrently without specialist oversight.

Oral corticosteroids do not alter alendronate pharmacokinetics directly, but they increase fracture risk independently and may confound the clinical picture when a child on chronic steroids is started on alendronate [16].

What Do Prescribers and Families Need to Know About Administration in Young Children?

Administration technique is the single greatest practical challenge in children under 12. The 30-minute upright requirement and plain-water-only rule are non-negotiable safety requirements, not preferences. A 2014 survey of pediatric endocrinology practices published in Hormone Research in Paediatrics found that 23% of pediatric patients on oral bisphosphonates had at least one documented administration error in the first 3 months of therapy, most commonly taking the medication with juice or milk [17].

Families should be trained at the first prescription visit. The child must take the tablet upon waking, before eating or drinking anything except plain water, with 180 to 240 mL of water, remain seated or standing for 30 minutes, and then eat breakfast. No lying down, no other liquids, and no other oral medications should be given during that window. If a weekly dose is missed, it should be taken the next morning and then returned to the once-weekly schedule; do not double-dose [1].

For children with OI who also have swallowing difficulties or developmental delay, the treating team may need to evaluate whether intravenous bisphosphonate therapy (pamidronate or zoledronic acid) is more appropriate than oral alendronate, given the lower administration burden and more predictable pharmacokinetics [12].

How Does Alendronate Compare to Other Bisphosphonates in Children Under 12?

Intravenous pamidronate has a longer track record in children under 12 than alendronate, largely because it bypasses the oral administration challenges and has been used in pediatric OI since the 1990s. A 2013 meta-analysis of bisphosphonate trials in pediatric OI (12 trials, N=819) found no statistically significant difference in lumbar spine BMD gains between intravenous pamidronate and oral alendronate at 12 months (weighted mean difference 0.3%, 95% CI minus 0.8 to 1.4%, P=0.58) [18]. Fracture data were too heterogeneous for pooling.

Zoledronic acid, given as a single annual intravenous infusion of 0.05 mg/kg (maximum 4 mg), has grown in use for pediatric OI and secondary osteoporosis. A 2021 RCT published in the Journal of Bone and Mineral Research (N=65, ages 1 to 17 years) found zoledronic acid non-inferior to pamidronate for lumbar spine BMD z-score change at 12 months [19]. Its convenience advantage is significant for young children, but it carries acute-phase reactions in roughly 40% of first-dose recipients and requires intravenous access.

Alendronate remains the preferred oral option when IV access is not feasible, when the family has logistical barriers to infusion-center visits, or when the treating specialist judges that oral therapy is sufficient for the degree of skeletal fragility present. The Cochrane review concluded that "the choice between oral and intravenous bisphosphonates in children should be individualized based on fracture severity, adherence capacity, and patient preference." [12]

What Is the Long-Term Safety Picture After Stopping Alendronate in Children?

Bisphosphonates accumulate in bone matrix and continue to be released slowly after discontinuation, a phenomenon called the "bisphosphonate tail." In children, this tail may extend the duration of biologic effect beyond the treatment period. Modeling studies suggest that in young patients, skeletal bisphosphonate retention could extend pharmacologic effects for 8 to 10 years post-treatment, though the clinical significance of this is not well-studied [20].

The primary concern after stopping alendronate in a child is rebound bone loss. Data from adult populations show that BMD gains during alendronate therapy are largely maintained for 3 to 5 years after discontinuation in adults who took the drug for 5 years (FLEX trial data) [14]. In children, particularly those in rapid growth phases, the remodeling-suppression tail may interact with the accelerated turnover of adolescence in ways that are difficult to predict.

Monitoring should continue for at least 12 months after stopping alendronate. Serum CTX typically rises to near-baseline within 6 to 12 months of discontinuation, serving as a biochemical marker that the bisphosphonate effect is waning [13]. If serum CTX normalizes and BMD z-scores remain stable or improve, no restart is required. If BMD z-scores decline and fractures recur, re-evaluation by a pediatric metabolic bone specialist is warranted.