Fosamax (alendronate) is one of the most widely prescribed osteoporosis drugs, and while most people tolerate it without serious problems, the worst side effects include damage to the esophagus, jawbone death, and unusual fractures of the thigh bone. These rare but severe complications are the reason Fosamax comes with strict dosing instructions and why doctors reassess the need for it after five years.
Esophageal Damage
The most common serious side effect of Fosamax involves the esophagus, the tube connecting your throat to your stomach. The drug can directly irritate the lining of the esophagus, and stomach acid containing dissolved alendronate can reflux upward and make things worse. Reported problems range from inflammation and ulcers to erosions that sometimes bleed. In rare cases, the damage progresses to scarring that narrows the esophagus or even perforation, a hole through the esophageal wall.
This is why Fosamax has very specific dosing rules that aren’t optional. You need to swallow the tablet with a full glass of plain water (at least 200 mL, about 7 ounces), first thing in the morning. Don’t chew or suck on the tablet. Don’t eat anything for at least 30 minutes afterward. And critically, don’t lie down for at least 30 minutes after taking it, and not until after you’ve eaten. Taking it at bedtime or before getting out of bed in the morning is specifically warned against. If you have any condition that slows the passage of food through your esophagus, such as a stricture, Fosamax is not an option for you.
Osteonecrosis of the Jaw
Osteonecrosis of the jaw (ONJ) is the side effect that generates the most fear, and understandably so. It’s a condition where bone tissue in the jaw dies and doesn’t heal properly, often triggered by a tooth extraction or dental infection. The exposed, dead bone can cause pain, swelling, and infection that’s difficult to treat.
The important context is how rarely this happens at osteoporosis doses. Cancer patients receiving very high intravenous doses of bisphosphonates face a risk of roughly 1 in 100. For people taking oral Fosamax at standard osteoporosis doses, the estimated risk drops dramatically to less than 1 in 60,000, and possibly as low as 1 in 200,000. That’s an enormous difference. The risk does increase with longer duration of use, which is one reason drug holidays are recommended. If you’re planning dental surgery, let your dentist know you take Fosamax so they can plan accordingly.
Atypical Thigh Bone Fractures
This side effect is particularly ironic: a drug taken to prevent fractures can, over time, cause an unusual type of fracture. These atypical fractures occur in the shaft of the femur (thigh bone), typically from minimal trauma or even just standing. They’re different from the hip fractures osteoporosis causes, which happen near the top of the bone.
The risk increases after five or more years of continuous use. That’s because bisphosphonates work by suppressing the normal bone-remodeling cycle, and over many years, this can make bones more brittle in certain areas rather than stronger. There’s often a warning sign: weeks to months before a complete fracture, many patients experience thigh or groin pain, sometimes with stress fracture features visible on imaging. If you’ve been on Fosamax for several years and develop new pain in your thigh, hip, or groin, that symptom needs to be evaluated promptly with imaging to check for an incomplete fracture before it becomes a complete one.
Severe Musculoskeletal Pain
In clinical trials, about 4% of patients on Fosamax developed muscle, bone, or joint pain, compared to 2.5% on placebo. That modest difference in trials doesn’t capture the full picture. Post-marketing reports have described severe and occasionally incapacitating bone, joint, or muscle pain in some patients. The FDA’s prescribing information specifically notes this and advises stopping the drug if severe symptoms develop. The pain can begin days to years after starting treatment and typically resolves after discontinuation.
Eye Inflammation
A lesser-known side effect is inflammatory eye disease, particularly uveitis (inflammation inside the eye) and scleritis (inflammation of the white outer coating). These weren’t flagged in clinical trials but emerged from post-marketing surveillance. In a case series of 18 patients who developed uveitis while on bisphosphonates, 61% were taking alendronate. The onset averaged about two and a half years after starting treatment, though it can appear much sooner, within days to months.
Symptoms typically include eye pain and redness, usually in one eye. More than half of affected patients in that series developed complications including cataracts, adhesions inside the eye, and macular edema. One large epidemiologic study estimated the prevalence at roughly 29 per 10,000 patient-years. While still uncommon, it’s worth knowing that new eye pain or redness during Fosamax treatment isn’t something to dismiss.
Low Calcium Levels
Fosamax works by blocking the cells that break down bone. Since bone breakdown normally releases calcium into the bloodstream, suppressing that process can lower calcium levels. For most people with adequate calcium and vitamin D intake, this isn’t a problem. But if you already have low calcium, vitamin D deficiency, or a condition that impairs mineral absorption (like celiac disease), Fosamax can push calcium levels low enough to cause symptoms. Existing calcium deficiency must be corrected before starting the drug, and vitamin D deficiency should be treated as well.
Why Doctors Recommend Drug Holidays
Because the worst side effects of Fosamax are tied to long-term use, current guidelines recommend reassessing treatment after five years for oral bisphosphonates like alendronate. At that point, patients at lower risk of fracture may take a “drug holiday,” stopping the medication while monitoring bone density annually. Fosamax has the unusual property of lingering in bone tissue for years after you stop taking it, so its protective effects don’t vanish immediately.
Treatment is typically restarted if bone density drops significantly, if your T-score falls back to -2.5 or below, or if you experience a new fracture. Patients who still have very low bone density after five years of treatment, or who have a history of hip or spine fractures, may be advised to continue rather than take a break. The decision balances the ongoing fracture-prevention benefit against the accumulating risk of atypical fractures and jawbone problems.
Who Should Not Take Fosamax
Fosamax is not recommended for anyone with kidney function below a creatinine clearance of 35 mL/min, as it hasn’t been studied in that population and the kidneys are responsible for clearing the drug. It’s also contraindicated in people with esophageal abnormalities that slow swallowing, anyone unable to sit or stand upright for 30 minutes, and those with uncorrected low calcium levels. Between a creatinine clearance of 35 and 60 mL/min, no dose adjustment is needed, but kidney function should be part of the conversation before starting treatment.