Cilostazol carries an FDA black box warning, the most serious type of drug safety alert, stating it is contraindicated in patients with heart failure of any severity. The reason comes down to how the drug works: it blocks an enzyme found in heart muscle cells, and other drugs that block that same enzyme have increased the risk of death in heart failure patients. Even though cilostazol itself hasn’t been tested in large heart failure trials, the FDA applied the contraindication as a class-wide precaution based on the deadly track record of similar drugs.
How Cilostazol Works in the Body
Cilostazol is prescribed to improve walking distance in people with peripheral artery disease who experience leg pain during exercise. It does this by inhibiting an enzyme called phosphodiesterase III (PDE3), which is found in platelets, blood vessel walls, and heart muscle tissue. When PDE3 is blocked, levels of a signaling molecule called cAMP rise inside cells. In blood vessels, this causes relaxation and improved blood flow. In platelets, it reduces clotting.
The problem is that PDE3 doesn’t just exist in blood vessels and platelets. It’s concentrated in the sarcoplasmic reticulum of heart muscle cells, where it helps regulate how forcefully the heart contracts. When cilostazol blocks PDE3 in the heart, rising cAMP levels trigger a chain reaction: a protein called protein kinase A is activated, which ultimately increases the force and rate of heart contractions. In a healthy heart, this might not cause obvious harm. In a failing heart, these effects become dangerous.
Why a Stronger Heartbeat Isn’t Always Better
It sounds counterintuitive that making a weak heart beat harder could be harmful. But in heart failure, the heart muscle is already under enormous stress. Forcing it to contract more powerfully increases its oxygen demand at a time when it may already be struggling to meet its own metabolic needs. Over time, this mismatch accelerates damage to the heart muscle rather than helping it.
The greater concern is electrical instability. Elevated cAMP levels in damaged heart cells can trigger abnormal electrical impulses known as early and delayed after-depolarizations. These are the cellular events that can spiral into ventricular tachycardia or ventricular fibrillation, both life-threatening rhythm disturbances. A heart weakened by failure is already prone to these arrhythmias, and PDE3 inhibition raises that risk further.
The Trial That Changed Everything
The strongest evidence behind this contraindication doesn’t come from cilostazol directly. It comes from a related PDE3 inhibitor called vesnarinone, tested in the Vesnarinone Survival Trial (VEST). This large study enrolled more than 3,800 patients with severe heart failure and randomized them to receive vesnarinone at two different doses or a placebo.
The results were striking. In the higher-dose group (60 mg), 22.7% of patients died compared to 18.6% in the placebo group, a statistically significant difference. The lower-dose group fared only slightly better at 20.9%. The annualized mortality rate in the 60 mg group was 29.1 per 100 patients, versus 23.5 in the placebo group. The excess deaths were driven specifically by sudden cardiac death, consistent with the arrhythmia risk that PDE3 inhibition creates in failing hearts.
Vesnarinone wasn’t the only PDE3 inhibitor to show this pattern. Milrinone, another drug in the same class, similarly increased mortality when used long-term in heart failure patients. The consistent signal across multiple drugs in this class is what led the FDA to treat the risk as inherent to the mechanism of action, not unique to any single compound.
The FDA’s Black Box Warning
The FDA’s prescribing information for cilostazol (brand name Pletal) opens with a boxed warning: “Cilostazol and several of its metabolites are inhibitors of phosphodiesterase III. Several drugs with this pharmacologic effect have caused decreased survival compared to placebo in patients with class III-IV heart failure.” The contraindication applies to heart failure of any severity, not just advanced stages. The National Institutes of Health drug labeling reinforces this with blunt patient-facing language: “Do not take cilostazol if you have heart failure of any kind.”
This is what’s known as a class-effect contraindication. The FDA didn’t wait for cilostazol-specific mortality data in heart failure patients. Given that the drug shares the exact same mechanism responsible for increased deaths with other PDE3 inhibitors, and given that those deaths were caused by sudden cardiac events directly linked to that mechanism, the agency applied the restriction preemptively. The 2024 ACC/AHA guidelines for peripheral artery disease echo this position, rating cilostazol use in heart failure patients as a Class 3 recommendation, meaning it should not be used.
Alternatives for Leg Pain With Heart Failure
If you have both peripheral artery disease and heart failure, cilostazol is off the table. That limits your pharmacological options for leg symptoms specifically, since cilostazol is the only medication with strong evidence for improving claudication symptoms and walking distance.
Statins, which most people with peripheral artery disease are already taking for cardiovascular risk reduction, have some evidence suggesting they can improve walking distance and blood vessel function in diseased leg arteries. ACE inhibitors, commonly prescribed for heart failure itself, also reduce long-term cardiovascular risk in peripheral artery disease patients, though they don’t directly relieve leg symptoms. The reality is that most drug therapy for peripheral artery disease focuses on preventing heart attacks and strokes rather than improving leg pain.
Supervised exercise programs remain one of the most effective non-drug approaches for improving claudication symptoms. For people whose symptoms are severe and don’t respond to conservative measures, procedures to restore blood flow in the legs (angioplasty, stenting, or bypass surgery) may be appropriate. These decisions depend heavily on the severity of both your heart failure and your peripheral artery disease, so the treatment path varies considerably from person to person.