Retained water, not lost salt
Drug-induced SIADH: the sodium falls and the tumour takes the blame
Hyponatremia is the commonest electrolyte disorder in oncology and its commonest explanation is the cancer itself — which is exactly why a drug that impairs free-water excretion can go on being given for months while the sodium is treated as a feature of the disease.
- 47%
- Cancer admissions with hyponatremiaAcross 4,702 admissions in 3,357 patients at a comprehensive cancer centre, sodium was below 135 mEq/L in nearly half — mild in 36%, moderate in 10%, severe in 1%.PMID 22001181
- 24%
- Acquired during the admissionA quarter of the hyponatremia was not present on arrival but developed in hospital, which is where the drugs are given.PMID 22001181
- 52%
- Incidence on high-dose cyclophosphamideIn a retrospective cohort of 69 adults receiving high-dose cyclophosphamide, cumulative incidence of hyponatremia was 52% (95% CI 39-64); severe 5.8%, symptomatic 8.7%.PMID 29890094
- HR 4.74
- 90-day mortality, moderate hyponatremiaAgainst eunatremic admissions, adjusted 90-day mortality hazard was 2.04 for mild, 4.74 for moderate and 3.46 for severe hyponatremia — this is not an incidental laboratory finding.PMID 22001181
Teaching case · illustrative composite, not a real patient
A 58-year-old woman receives high-dose cyclophosphamide. Her sodium is 139 mEq/L before the infusion. Thirty-six hours later she is nauseated and confused, and the sodium is 124 mEq/L. She is clinically euvolemic — no oedema, no orthostasis, normal jugular venous pressure — and her urine output has barely changed. Serum osmolality is low, urine osmolality is inappropriately high, and urine sodium is not low. She has small-volume metastatic breast disease with no brain or lung involvement.
The reflex reading is paraneoplastic SIADH from the malignancy. Against that: the fall is abrupt, dated to the infusion rather than to the disease, and her sodium was normal two days earlier. Hypotonic maintenance fluid is stopped and free water restricted. The sodium rises over the following days without hypertonic saline and settles back to baseline before the next cycle. A vasopressin level, had it been sent, would not have settled the question — cyclophosphamide can produce this picture with vasopressin suppressed.
Teaching point — In a patient with cancer, hypotonic euvolemic hyponatremia has a default explanation that is usually right and occasionally expensive. What distinguishes the drug is timing: a sodium that was normal before the dose and falls within hours to days of it, then recovers on withdrawal, is behaving like a drug effect regardless of how plausible the tumour is. Establish the temporal relation before accepting the paraneoplastic answer, because that answer is the one that lets the exposure continue.
How it happens
The pathophysiology as a cascade — select a step to follow the mechanism.
In SIADH the kidney handles sodium normally — external sodium balance stays regulated — and the abnormality is retained free water. That is why the patient looks euvolemic rather than oedematous, why urine output changes little, and why giving salt without addressing water does not fix it.
PMID 24513602 (opens PubMed in a new tab)Vincristine and ifosfamide are associated with sustained plasma vasopressin — classical SIADH, with the hormone genuinely elevated for the prevailing osmolality.
PMID 36233678 (opens PubMed in a new tab)Cyclophosphamide upregulates the V2 receptor and aquaporin-2 inside the collecting duct without any vasopressin present, so plasma vasopressin is SUPPRESSED by negative feedback. In rat inner medullary collecting duct cells the effect ran through V2R-cAMP-protein kinase A signalling and was abolished by tolvaptan or a PKA inhibitor. This is nephrogenic SIAD, and the review concludes it is the major mechanism of drug-induced hyponatremia.
PMID 36233678 (opens PubMed in a new tab)Measuring the hormone distinguishes the two routes; it does not rule the drug in or out. A suppressed vasopressin is the EXPECTED finding in the nephrogenic form, not evidence against it — which is the practical reason the diagnosis rests on timing and on the water handling rather than on an assay.
PMID 36233678 (opens PubMed in a new tab)In the vinblastine case that defined the severe end, markedly raised urinary tubular enzymes accompanied the hyponatremia and impaired reabsorption of sodium and coupled solutes, implicating a tubular lesion in addition to inappropriate ADH release. Where a salt-losing component is present, water restriction alone is the wrong treatment.
PMID 2452936 (opens PubMed in a new tab)
How we learned it
- 1985
High-dose intravenous melphalan is reported to cause SIADH
Both children given 2 mg/kg developed hyponatremia at 124-125 mEq/L with inappropriate urinary sodium losses, and 7 of 10 at 1 mg/kg trended downward — a previously unreported complication of the drug, and an early demonstration of dose dependence.
PMID 3965085 (opens PubMed in a new tab) - 1988
Vinblastine produces hyponatremia at 104 mEq/L with measurable ADH
Circulating ADH was detectable despite the lowest plasma osmolality — the definitional inappropriateness — and urinary tubular enzymes were markedly raised, implicating a tubular lesion alongside the hormone.
PMID 2452936 (opens PubMed in a new tab) - 2002
Vincristine hyponatremia is characterised across a global safety database
76 cases at an estimated 1.3 per 100,000 treated patients, three-quarters of them being treated for leukemia or lymphoma — rare, but reversible and serious enough to warrant recognition.
PMID 12051122 (opens PubMed in a new tab) - 2012
Hyponatremia in cancer is tied to length of stay and mortality
The finding that moved it from a monitored number to an outcome: 47% of admissions affected, with adjusted 90-day mortality hazards up to 4.74 and stays roughly twice as long.
PMID 22001181 (opens PubMed in a new tab) - 2022
Drug-induced hyponatremia is split into two mechanisms
Vincristine and ifosfamide track with sustained vasopressin — true SIADH — while cyclophosphamide acts on the collecting duct itself with vasopressin suppressed. The same electrolyte, two different lesions, and only one of them is detectable by measuring the hormone.
PMID 36233678 (opens PubMed in a new tab)
The landmark studies
Hyponatremia in hospitalized cancer patients and its impact on clinical outcomes
Doshi SM et al. · Am J Kidney Dis 2012 · PMID 22001181
Hyponatremia affected nearly half of cancer admissions and was independently associated with longer hospital stay and higher 90-day mortality at every severity, including mild. Whether correcting it changes those outcomes was left open.
47% of admissions (mild 36%, moderate 10%, severe 1%); acquired in hospital in 24%; mean stay 5.6 d eunatremic vs 9.9 / 13.0 / 11.5 d; 283 deaths (8.4%); 90-day mortality HR 2.04 (1.42-2.91) / 4.74 (3.21-7.01) / 3.46 (1.05-11.44)
Pathophysiology of Drug-Induced Hyponatremia
Kim GH · J Clin Med 2022 · PMID 36233678
Separates two mechanisms that present identically. Vincristine and ifosfamide are associated with sustained plasma vasopressin and cause true SIADH; cyclophosphamide, along with several psychotropics and thiazides, upregulates the V2 receptor and aquaporin-2 within the collecting duct in the ABSENCE of vasopressin — nephrogenic SIAD, in which plasma vasopressin is suppressed by negative feedback. The author concludes nephrogenic antidiuresis is the major mechanism of drug-induced hyponatremia.
In rat inner medullary collecting duct cells, cyclophosphamide (with haloperidol, sertraline, carbamazepine) upregulated V2R mRNA and raised cAMP without vasopressin; the resulting aquaporin-2 upregulation was blocked by tolvaptan and by protein kinase A inhibitors
Diagnosis and Management of Hyponatremia: A Review
Adrogué HJ et al. · JAMA 2022 · PMID 35852524
Sets the correction arithmetic. Severely symptomatic hyponatremia is a medical emergency treated with bolus hypertonic saline; the ceiling on total correction is what prevents osmotic demyelination, and it is exceeded often enough that it has to be actively managed rather than assumed.
Affects ~5% of adults and ~35% of hospitalized patients; US and European guidelines advise raising sodium 4-6 mEq/L within 1-2 h for severe symptoms but by no more than 10 mEq/L in the first 24 h; that limit is exceeded in about 4.5-28% of people
[Hyponatremia induced by high-dose cyclophosphamide therapy: a retrospective cohort study Cyclophosphamide and Hyponatremia]
Bonella BM et al. · Rev Fac Cien Med Univ Nac Cordoba 2017 · PMID 29890094
Cumulative incidence was far higher than earlier reports had suggested, which the authors attribute to restricting the cohort to high-dose therapy. Severe and symptomatic hyponatremia were uncommon, but a fifth of patients had their admission prolonged by it.
69 patients; hyponatremia 52% (95% CI 39-64); severe (<120 mEq/L) 5.8% (0-12); symptomatic 8.7% (1.3-16); female sex the only independent association, OR 3.89 (1.02-8.55), p=0.04
Safety and Efficacy Analysis of Selinexor-Based Treatment in Multiple Myeloma, a Meta-Analysis Based on Prospective Clinical Trials
Tao Y et al. · Front Pharmacol 2021 · PMID 34925019
Hyponatremia is a leading non-hematological toxicity of selinexor rather than a rare one, and its frequency depends heavily on the partner regimen — adding a proteasome inhibitor to selinexor-dexamethasone cut it roughly threefold at every grade while improving response.
Selinexor+dexamethasone vs selinexor+dexamethasone+proteasome inhibitor: any-grade hyponatremia 39% vs 12% (p<0.00001); grade ≥3 22% vs 5% (p<0.0001)
Hyponatremia and syndrome of inappropriate anti-diuretic hormone reported with the use of Vincristine: an over-representation of Asians?
Hammond IW et al. · Pharmacoepidemiol Drug Saf 2002 · PMID 12051122
Establishes the vincristine association as real but uncommon, and raises a possible ancestry signal that has never been resolved. The authors are explicit that the overall rate is very low and that the event is serious but reversible.
76 cases; reporting rate ~1.3 per 100,000 treated patients; mean age 35.6 ± 28.3 y; 62% male; ~75% treated for leukemia or lymphoma; of 39 reports stating race, 35 Asian, 3 Caucasian, 1 Black
What the data says now
How disproportionately each agent's FAERS reports name these phenotypes vs. all other drugs (reporting odds ratio; significant signals only, 95% CI lower bound > 1; as of 2026-10-01). A reporting signal, not incidence or proven causation. A dash means tested without reaching significance, not a phenotype that never occurs — ATN and AIN undercount badly, most true cases filing as generic “acute kidney injury”. Computed by this atlas on the current snapshot — a published disproportionality analysis will not match cell for cell (different window, different term set).
| Agent | SIADH |
|---|---|
| Cyclophosphamide | Cyclophosphamide, SIADH / Hyponatremia: ROR 2.35, 1,565 reports |
| Vincristine | Vincristine, SIADH / Hyponatremia: ROR 3.48, 384 reports |
| Vinblastine | Vinblastine, SIADH / Hyponatremia: ROR 4.49, 33 reports |
| Vinorelbine | Vinorelbine, SIADH / Hyponatremia: ROR 1.98, 52 reports |
| Melphalan | Melphalan, SIADH / Hyponatremia: ROR 1.95, 186 reports |
| Selinexor | Selinexor, SIADH / Hyponatremia: ROR 8.59, 275 reports |
Hyponatremia among cancer admissions
3,357 patients admitted to a comprehensive cancer centre over three months
PMID 22001181 (opens PubMed in a new tab)Hyponatremia acquired during the hospital stay
Same cohort
PMID 22001181 (opens PubMed in a new tab)Incidence on high-dose cyclophosphamide
69 adults receiving high-dose cyclophosphamide, 2010-2014
PMID 29890094 (opens PubMed in a new tab)Hyponatremia on selinexor, by partner regimen
477 patients with multiple myeloma across six prospective trials
PMID 34925019 (opens PubMed in a new tab)Vincristine hyponatremia/SIADH reporting rate
Global manufacturer safety database through November 1999
PMID 12051122 (opens PubMed in a new tab)Melphalan hyponatremia by dose
12 children given intravenous bolus melphalan for resistant marrow-involving tumours
PMID 3965085 (opens PubMed in a new tab)How it's managed
- 1
Confirm the water story before treating the number
The diagnosis needs hypotonic plasma, clinical euvolemia, and urine that is inappropriately concentrated with sodium that is not low. Getting this right is what separates SIADH from hypovolemic and hypervolemic hyponatremia, which are managed in opposite directions — and the volume assessment is the step most often skipped.
SIADH is a pure disorder of renal water handling with external sodium balance preserved · PMID 24513602 (opens PubMed in a new tab)
- 2
Treat severe symptoms as an emergency, on the clock
Somnolence, obtundation, seizures or cardiorespiratory distress call for bolus hypertonic saline to raise the sodium by 4-6 mEq/L within 1-2 hours. The target is reversal of encephalopathy, not normalisation of the sodium.
US and European guidance as summarised in a JAMA review · PMID 35852524 (opens PubMed in a new tab)
- 3
Hold the 24-hour ceiling deliberately
Correction must not exceed 10 mEq/L in the first 24 hours. That limit is breached in roughly 4.5-28% of patients, and overly rapid correction of chronic hyponatremia causes osmotic demyelination — rare, but capable of leaving parkinsonism, quadriparesis or death. Plan the re-lowering strategy before you start, not after the sodium overshoots.
Reported overcorrection frequency and its consequence · PMID 35852524 (opens PubMed in a new tab)
- 4
For the drug-induced cases, restriction is often enough
Where the patient is not severely symptomatic, removing free water and stopping the offending exposure has repeatedly been sufficient without hypertonic saline: a vinorelbine case corrected in a couple of days on fluid restriction alone, and an osimertinib case improved substantially within a week of restriction plus drug withdrawal.
Two case reports, different drug classes, same response · PMID 17989977 (opens PubMed in a new tab)
- 5
Withdrawal is not always the end of the drug
When the implicated agent is the best or only option, it may not have to be abandoned. Osimertinib was resumed at a reduced dose without recurrence of SIADH in a patient with a T790M mutation and no effective alternative; in another case a within-class switch to erlotinib was tolerated with no hyponatremia. Both are single cases — the point is that permanent discontinuation is a decision, not an automatic consequence.
Case reports of dose-reduced rechallenge and of class switch · PMID 41132965 (opens PubMed in a new tab)
- 6
Separate the drug from the tumour, and expect both
Ectopic ADH release — classically small-cell lung cancer — is the competing diagnosis, and central nervous system disease and pneumonia produce the same picture. None of these excludes a drug effect in the same patient. Temporal relation to the dose and resolution on withdrawal are what settle attribution, and getting it wrong in the tumour's favour means the exposure continues.
SIADH in oncology has multiple simultaneous causes, drugs among them · PMID 24513602 (opens PubMed in a new tab)
Every citation on this page is a real, PubMed-verified reference. The teaching case is an illustrative composite, not a real patient. Medical-education content — not medical advice.