Low blood pressure in cancer patients stems from a wide range of causes, and the answer is rarely just one thing. The cancer itself, the treatments used against it, infections that take hold when the immune system is suppressed, and bleeding from tumors can all drive blood pressure down. Research suggests cancer patients are about twice as likely to develop orthostatic hypotension as people without cancer, and the risk shifts depending on the type of cancer and the treatments involved.1MDPI (Cancers). The Prevalence of Orthostatic Hypotension in Cancer Patients Understanding these causes matters because some are easily fixable, while others signal a medical emergency.
Dehydration and Volume Loss
The single most common reason cancer patients develop low blood pressure is straightforward: they don’t have enough fluid in their bloodstream. Cancer and its treatments attack the body’s ability to stay hydrated from several angles. Chemotherapy frequently causes nausea, vomiting, and diarrhea, all of which drain fluid faster than most patients can replace it through drinking. Radiation therapy to the head, neck, or abdomen can cause painful mouth sores and inflamed intestinal lining, making eating and drinking difficult or even agonizing. Many patients simply lose their appetite or develop aversions to food and water.
Beyond losing fluid through the gut, cancer patients can also lose volume through a more subtle route. In advanced disease, especially when the GI tract is blocked by a tumor, fluid can shift out of the bloodstream and into the spaces between cells, a phenomenon sometimes called “third-spacing.” A study of terminally ill cancer patients with intestinal obstruction found that even patients receiving intravenous hydration experienced intravascular volume depletion because fluid was leaking from the bloodstream into surrounding tissues.2SpringerLink (Supportive Care in Cancer). Fluid status of terminally ill cancer patients with intestinal obstruction: an exploratory observational study The result is that a patient’s blood pressure drops even though their body is retaining fluid overall, because that fluid is in the wrong compartment.
Chemotherapy, Radiation, and Direct Drug Toxicity
Many cancer treatments lower blood pressure as a direct pharmacological side effect, not just through dehydration. Vincristine, a chemotherapy drug used for lymphomas and other cancers, can damage the autonomic nerves that regulate blood pressure. One documented case involved a patient with malignant lymphoma who developed severe orthostatic hypotension during vincristine therapy. The symptoms resolved once the drug was stopped, confirming the nerve damage was reversible in that case.3Karger (Onkologie). Reversible orthostatic hypotension during vincristine therapy
Interleukin-2 (IL-2), an immune-stimulating drug used for melanoma and kidney cancer, is notorious for causing severe hypotension through vascular leak syndrome. IL-2 triggers the inner lining of blood vessels to become leaky, allowing plasma to seep out of the bloodstream and into surrounding tissues. Research has shown that albumin concentration drops significantly more than blood volume increases during IL-2 therapy, confirming that protein-rich fluid is literally leaking through capillary walls.4PubMed. Determination of capillary leakage due to recombinant interleukin-2 by means of noninvasive conductivity measurements The mechanism involves a specific enzyme in blood vessel walls, and the resulting blood pressure drop can be severe enough to require intensive care.5PubMed. Endothelial nitric oxide synthase is a key mediator of interleukin-2-induced hypotension and vascular leak syndrome
Radiation therapy also carries its own risk. In one study comparing hypotension rates among cancer patients receiving different treatments, those undergoing radiation therapy had a higher rate of orthostatic hypotension than those on chemotherapy alone, roughly 43% versus 24%.1MDPI (Cancers). The Prevalence of Orthostatic Hypotension in Cancer Patients The reasons for this are not entirely pinned down but likely relate to radiation’s ability to damage small blood vessels and autonomic nerves in the treatment field, combined with the fatigue and reduced intake that often accompany radiation courses.
Immunotherapy and Cytokine Release Syndrome
Newer immunotherapy drugs have transformed cancer treatment but introduced their own set of blood-pressure-lowering complications. The most dramatic is cytokine release syndrome, or CRS, which occurs when immune cells activated by treatment release massive quantities of inflammatory signaling molecules into the bloodstream. CRS is best known as a side effect of CAR-T cell therapy, where a patient’s own immune cells are genetically reprogrammed to attack cancer. It typically starts with a fever and can progress to life-threatening capillary leak with low oxygen levels and severe hypotension.6Biology of Blood and Marrow Transplantation. Cytokine Release Syndrome with Chimeric Antigen Receptor T Cell Therapy
Checkpoint inhibitors like pembrolizumab and nivolumab cause a different, more insidious problem. These drugs work by removing the brakes on the immune system, but occasionally the immune system then attacks the body’s own hormone-producing glands. When the pituitary gland or adrenal glands are targeted, the result is adrenal insufficiency: the body can no longer produce cortisol, the hormone that helps maintain blood pressure. Without cortisol, blood pressure can plummet and may not respond to standard fluid resuscitation.
In one case, a woman with triple-negative breast cancer developed persistent hypotension, rapid heart rate, and rapid breathing four months after starting pembrolizumab. Her morning cortisol level was nearly undetectable, and she was diagnosed with secondary adrenal insufficiency.7PubMed Central. Immunotherapy Induced Adrenal Insufficiency: An Underdiagnosed Cause of Persistent Hypotension in Cancer Similarly, a man with lung cancer on nivolumab arrived at the emergency department in shock, with blood pressure of 68/48 mmHg. His cortisol levels, which had been normal two months earlier, were now critically low. He recovered quickly once doctors gave him intravenous hydrocortisone.8PubMed Central. Refractory hypotension due to Nivolumab-induced adrenal insufficiency The key clinical lesson in both cases is that the hypotension was “refractory,” meaning it didn’t improve with IV fluids alone. Recognizing adrenal insufficiency as the cause and giving steroids made the difference.
Infusion Reactions and Allergic Responses
Any time a cancer drug is given intravenously, there is a risk of an infusion-related reaction. These reactions typically occur within minutes to hours after the infusion starts and range from mild flushing to severe anaphylaxis-like events.9PubMed Central. Management of infusion-related reactions in cancer therapy: strategies and challenges Symptoms of acute hypersensitivity reactions to chemotherapy agents include flushing, nausea, difficulty breathing, back pain, low blood pressure, and a racing heart.10PubMed. Acute hypersensitivity reactions to chemotherapy agents: an overview
Some drugs are more prone to causing these reactions than others. Taxanes, platinum compounds, and monoclonal antibodies are among the frequent offenders. The reactions can happen on the first exposure or develop after several uneventful infusions. In a severe reaction, blood vessels dilate rapidly and fluid shifts out of the circulation, causing a sharp drop in blood pressure that requires immediate intervention with epinephrine and aggressive fluid support. Infusion centers are set up to catch these events early, which is why patients are typically monitored closely during and after their treatments.
Infections and Sepsis
Cancer patients, especially those undergoing chemotherapy, frequently develop neutropenia, a dangerously low white blood cell count that leaves them vulnerable to infections. When an infection takes hold in a neutropenic patient, it can escalate to sepsis rapidly. In sepsis, the body’s inflammatory response to infection causes blood vessels throughout the body to dilate, and fluid leaks from the bloodstream into tissues. The result is a rapid, sometimes catastrophic drop in blood pressure.
Research on febrile episodes in neutropenic cancer patients identified hypotension and shock as the most significant predictors of poor outcomes.11PubMed. Evaluation of infectious etiology and prognostic risk factors of febrile episodes in neutropenic cancer patients This is why any cancer patient on chemotherapy who develops a fever is treated as a medical urgency. The fever itself might seem minor, but in a patient with essentially no functioning immune defense, it can be the first sign of an infection spiraling toward septic shock. Getting antibiotics started within the first hour dramatically improves survival.
Bleeding From Tumors
Tumors, particularly those in the gastrointestinal tract, can erode into blood vessels and cause hemorrhage. The bleeding can be sudden and massive or slow and chronic. Either way, it reduces the volume of blood available to circulate, and blood pressure drops accordingly. Gastrointestinal tumors are an especially common source of acute hemorrhage because the gut has a rich blood supply and tumors growing into the intestinal wall can rupture into the lumen.
A large retrospective study of over 500 patients with acute gastrointestinal bleeding from GI tumors found that rapid vascular access and blood pressure recovery were critical to survival. Patients who received faster resuscitation had a lower 30-day mortality rate, about 12% compared with roughly 19% in the group with slower access.12PubMed Central. Intraosseous versus intravenous fluid resuscitation in gastrointestinal tumor-related acute hemorrhage: impact on 30-day mortality and lactate clearance Chronic, slow-drip bleeding is easier to miss. A patient might not notice blood in their stool for weeks, and by the time the low blood pressure surfaces as dizziness or fainting, they may already be profoundly anemic.
Tumors Pressing on Blood Vessels and the Heart
Cancer can physically block the circulation. A large tumor in the chest, for example, can compress the superior vena cava, the major vein that returns blood from the upper body to the heart. When blood can’t get back to the heart efficiently, the heart has less to pump out, and blood pressure falls. In one case, a patient became severely hypotensive after intubation because a lung mass was compressing the SVC and an intraluminal blood clot was further blocking flow.13PubMed. Hypotensive patient with superior vena cava obstruction diagnosed using resuscitative transesophageal echocardiography
Pericardial effusion, where fluid accumulates in the sac around the heart, is another mechanical cause. Many cancers can metastasize to the pericardium or cause inflammation that leads to fluid buildup. When enough fluid collects, it squeezes the heart and prevents it from filling properly, a condition called cardiac tamponade. Drainage through pericardiocentesis is needed when the effusion causes hemodynamic impairment.14Europe PMC. Pericardial effusion in oncological patients: current knowledge and principles of management Similarly, cancer patients face an elevated risk of blood clots, and a large pulmonary embolism can suddenly obstruct blood flow from the right side of the heart to the lungs. Shock and right ventricular failure from pulmonary embolism are associated with poor outcomes.15PubMed. Perioperative pulmonary embolism: diagnosis and anesthetic management
Unusual and Easily Missed Causes
Some causes of low blood pressure in cancer patients are rare enough that they can go unrecognized for weeks or longer. Carcinoid tumors, which arise from neuroendocrine cells most often in the gut or lungs, can secrete hormones like serotonin and histamine directly into the bloodstream. The resulting carcinoid syndrome includes flushing, diarrhea, airway spasm, and hypotension.16PubMed. The spectrum of carcinoid tumours and carcinoid syndromes Because these symptoms overlap with chemotherapy side effects, the syndrome can be mistaken for a drug reaction rather than a tumor-driven hormonal problem.
Head and neck cancers can cause hypotension through an entirely different mechanism. When a tumor in the neck encases or compresses the carotid sinus, the pressure-sensing structure at the fork of the carotid artery, it can trick the body into thinking blood pressure is too high. The brain responds by slowing the heart and dilating blood vessels, causing sudden drops in blood pressure and even fainting. One patient with laryngeal cancer experienced recurrent episodes of hypotension and dangerously slow heart rate because lymphedema and tumor growth encased both carotid arteries, triggering carotid sinus syndrome.17PubMed Central. Carotid Sinus Syndrome With Convulsive Syncope in a Patient With Head and Neck Cancer A separate case report documented a similar situation where CT imaging revealed direct tumor compression of the left carotid sinus.18PubMed Central. Carotid Sinus Syndrome in a Patient with Head and Neck Cancer: A Case Report
In blood cancers like multiple myeloma, abnormal proteins produced by cancer cells can deposit in nerves and organs as amyloid. When amyloid deposits damage the autonomic nervous system, the body loses its ability to regulate blood pressure during position changes. One case of amyloid light-chain amyloidosis involved progressively worsening orthostatic hypotension that eventually confined the patient to bed.19PubMed Central. Progressively invalidating orthostatic hypotension: A common symptom for a challenging diagnosis
Nutritional Deficiency as a Hidden Contributor
Malnutrition is almost universal in advanced cancer, and specific vitamin deficiencies can contribute to circulatory problems in ways that are easy to overlook. Thiamine (vitamin B1) is essential for energy metabolism in cells, including heart muscle cells. When thiamine is depleted, the heart weakens and blood vessels lose their ability to maintain tone, both of which lower blood pressure. The chemotherapy drug 5-fluorouracil (5-FU) has been shown to actively deplete thiamine, and when combined with poor food intake or parenteral nutrition without adequate vitamin supplementation, patients can develop a condition resembling beriberi. One report described a patient on 5-FU who developed severe multi-organ failure including cardiovascular instability, lactic acidosis, and nerve damage, all consistent with profound thiamine deficiency.20Europe PMC. Severe encephalopathy, lactic acidosis, vegetative instability and neuropathy with 5-Fluorouracil treatment – pyrimidine degradation defect or beriberi?
Thiamine deficiency is treatable cheaply and effectively, which makes it especially important to catch. The challenge is that the symptoms overlap with chemotherapy toxicity, so clinicians may attribute the cardiovascular instability to the cancer treatment rather than a correctable nutritional gap. Patients on prolonged courses of 5-FU, those receiving nutrition through a vein, and those who have been eating poorly for weeks are at highest risk.
Which Cancer Types Carry the Highest Risk
Not all cancers contribute equally to hypotension risk. In the study that found cancer patients were twice as prone to orthostatic hypotension as non-cancer patients, lung cancer stood out with the highest rate: nearly 58% of lung cancer patients had orthostatic hypotension. Breast cancer (about 38%), male genitourinary cancers (about 33%), and head and neck cancers (about 31%) followed.1MDPI (Cancers). The Prevalence of Orthostatic Hypotension in Cancer Patients Lung cancer’s prominence likely reflects the combination of tumor location near major blood vessels, high rates of autonomic dysfunction, and the aggressive systemic treatments these patients receive. Head and neck cancers, as discussed above, can directly interfere with blood pressure regulation through carotid sinus involvement.
Gastrointestinal cancers carry a particularly high risk of hemorrhage-driven hypotension because of their proximity to the gut’s blood supply. Blood cancers like myeloma can cause amyloidosis-related nerve damage, while neuroendocrine tumors have their own hormone-driven pathway. In practice, clinicians caring for cancer patients tend to keep a broad differential in mind because the cause of low blood pressure in any given patient is often multifactorial, with dehydration, drug effects, and disease progression all contributing simultaneously.
Low Blood Pressure in the Final Days of Life
For patients with advanced, terminal cancer, falling blood pressure is one of the clinical signs that indicate the body is shutting down. A prospective study tracking bedside signs in dying cancer patients found that systolic blood pressure below 100 mmHg was present in about 82% of patients during the last three days of life, with a median onset roughly four days before death. Diastolic blood pressure below 60 mmHg appeared in about 62% of patients during the final three days.21Wiley Online Library (Cancer). Bedside clinical signs associated with impending death in patients with advanced cancer: Preliminary findings of a prospective longitudinal cohort study
At this stage, the hypotension reflects a general decline in organ function rather than a single correctable cause. The heart muscle weakens, blood vessels lose the ability to constrict, and the regulatory systems that maintain circulation fail. For families and caregivers, understanding that dropping blood pressure in a dying patient is part of the natural process can help set realistic expectations. In palliative care, the goal shifts from correcting the blood pressure to keeping the patient comfortable, and aggressive interventions like vasopressor drugs are generally not appropriate unless they serve a clear comfort-focused purpose.
Why Multiple Causes Often Overlap
In clinical practice, the question is rarely “what is the single cause of this patient’s low blood pressure?” A person going through chemotherapy might be dehydrated from vomiting, mildly anemic from slow tumor bleeding, and taking a blood pressure medication that was prescribed before their cancer diagnosis and never adjusted. Add a fever from a brewing infection, and the blood pressure can collapse from the combined weight of factors that would each be manageable alone. This layering effect is one reason cancer patients can go from seemingly stable to critically hypotensive with little warning.
Medications the patient was already taking before their cancer diagnosis deserve special attention. Blood pressure drugs, diuretics, and certain antidepressants all lower blood pressure, and doses that were appropriate when the patient was eating normally and well-hydrated can become dangerously potent when cancer treatment suppresses appetite and causes fluid loss. Oncology teams frequently adjust or temporarily stop these medications during active treatment, but the conversation doesn’t always happen proactively. If you are a cancer patient or caregiver and notice persistent dizziness when standing, unusual fatigue, or lightheadedness, flagging it early gives your care team a chance to sort through the contributing factors before the situation becomes urgent.