When reviewing medication regimens in older adults, it is easy to focus on medications that increase fall risk, cause cognitive impairment, or contribute to drug interactions. However, some medications can quietly impact bone health over many years before the consequences become apparent. Let’s take a look at a clinical scenario involving long-term phenytoin use and osteoporosis.
Case Scenario
A 76-year-old female presents to her primary care provider after sustaining a wrist fracture from a minor fall. Her medical history includes epilepsy, hypertension, and hyperlipidemia. Her medication list includes:
- Phenytoin 300 mg daily (taken for over 20 years)
- Lisinopril 20 mg daily
- Atorvastatin 20 mg daily
Following the fracture, a bone mineral density scan reveals osteoporosis with a T-score of -2.8 at the femoral neck. Laboratory evaluation demonstrates a 25-hydroxyvitamin D level of 12 ng/mL.
Further review reveals no history of smoking, excessive alcohol use, chronic corticosteroid therapy, or other obvious secondary causes of osteoporosis. Could phenytoin be contributing to this patient’s bone disease?
How Phenytoin Affects Bone Health
Phenytoin is a potent enzyme-inducing anticonvulsant that accelerates the metabolism of vitamin D in the liver. Long-term use can increase the breakdown of vitamin D and ultimately cause concentrations to fall below desired levels (excellent board exam nugget). Low vitamin D levels can contribute to osteoporosis.
Why This Adverse Effect Is Often Missed
Unlike dizziness or sedation, bone loss develops slowly and silently. Many patients remain stable on phenytoin for decades. Because the medication has been part of the regimen for so long, clinicians may overlook its contribution to declining bone health. The first indication of a problem may be too late when a patient has a debilitating fracture. Periodic assessment of vitamin D levels is important in patients taking phenytoin, and supplementation is often necessary.
Additional Risk Factors
In this case, the patient’s age and sex further increase fracture risk. Postmenopausal women are already at increased risk for osteoporosis due to reduced estrogen levels. The addition of a medication that depletes vitamin D can further accelerate bone loss.
Recall some additional osteoporosis risk factors:
- Limited physical activity
- Inadequate dietary calcium intake
- Low sun exposure
- History of falls
- Frailty
- Smoking or excessive alcohol use
Monitoring Considerations
When monitoring phenytoin and osteoporosis risk, monitoring is critical. In addition to checking vitamin D levels, additional monitoring of calcium, bone mineral density, fracture history, and fall risk may all be important items to monitor. These considerations become especially important in older adults and postmenopausal women.
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