Oxybutynin and Hyponatremia
Oxybutynin does not typically cause hyponatremia as it is not among the common medications associated with this electrolyte abnormality.
Mechanism of Action and Side Effects of Oxybutynin
- Oxybutynin is a potent muscarinic receptor antagonist used to treat overactive bladder 1
- Common side effects of oxybutynin include dry mouth, constipation, blurred vision, dizziness, and impaired cognition, but not hyponatremia 1
- Transdermal formulations of oxybutynin have improved side effect profiles compared to oral formulations due to bypassing first-pass hepatic metabolism, but hyponatremia is not mentioned as a side effect 1
Medications Commonly Associated with Hyponatremia
- Diuretics, antidepressants, and antiepileptics are established causes of hyponatremia 2
- Other medications implicated in hyponatremia include newer antihypertensive agents, antibiotics, and proton pump inhibitors 2
- In a study of drug-induced hyponatremia, the following medications were significantly associated with hyponatremia: diuretics, ACE inhibitors/ARBs, anti-arrhythmics, and opioids 3
- Serotonin reuptake inhibitors were found in 9.8% of hyponatremic patients in a general practice study 3
Evidence Comparing Oxybutynin with Other Medications
- In a population-based cohort study comparing desmopressin to oxybutynin, the rate of hyponatremia was 146 per 1,000 person-years for desmopressin versus only 11 per 1,000 person-years for oxybutynin 4
- This study demonstrates that oxybutynin has a significantly lower risk of hyponatremia compared to desmopressin (HR 13.19; 95% CI 6.69,26.01; p < 0.01) 4
- Oxybutynin was actually used as a control medication in this study precisely because it is not known to cause hyponatremia 4
Causes and Management of Hyponatremia
- Hyponatremia (serum sodium <135 mmol/L) is the most common electrolyte abnormality in hospitalized patients 2
- Syndrome of Inappropriate ADH (SIADH) is a common cause of hyponatremia, particularly in patients with lung cancer 5
- Medications that can cause SIADH include diuretics, psychotropic drugs, and anticonvulsants, but antimuscarinic medications like oxybutynin are not typically listed 6
Clinical Implications and Monitoring
- Hyponatremia is associated with higher rates of hospitalization (13.7% vs 7.7%) and death (3.9% vs 1.8%) compared to normonatremic patients 3
- The likelihood of hyponatremia increases with polypharmacy, trebling when four drugs are taken and increasing seven-fold with six drugs 3
- For patients on medications that may cause hyponatremia, regular monitoring of serum sodium levels is recommended 3
Conclusion
Based on the available evidence, oxybutynin is not known to cause hyponatremia. In fact, it has been used as a control medication in studies examining medications that do cause hyponatremia. When evaluating causes of hyponatremia, other medications such as diuretics, antidepressants, antiepileptics, and certain antihypertensives should be considered before attributing it to oxybutynin.