Nursing care
Why nitroglycerin causes headache and low blood pressure
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated October 2026
Short answer
Nitroglycerin releases nitric oxide, which relaxes vascular smooth muscle. Widened veins reduce the blood returning to the heart, lowering its oxygen demand and relieving angina. The same vasodilation widens vessels in the head, causing headache, and can drop blood pressure, especially when upright. PDE5 inhibitors amplify the same pathway, so the combination can cause severe hypotension.
One action explains the benefit and the side effects
Nitroglycerin is converted to nitric oxide, which activates an enzyme in vascular smooth muscle that raises a messenger called cyclic GMP. Higher cyclic GMP relaxes the muscle and the vessel widens. The effect is strongest on veins, which pool more blood, with additional dilation of arteries and coronary vessels.
Pooling in the veins reduces preload, the volume of blood returning to the heart. A less-filled heart has lower wall tension and needs less oxygen, which relieves the supply and demand mismatch behind angina. Every main side effect of nitroglycerin is this same vasodilation showing up somewhere the nurse did not intend, so understanding the action predicts the adverse effects.
Why the head throbs and the pressure falls
Dilation of blood vessels in the head produces the typical throbbing headache, which is common and dose related, particularly when therapy starts. Consumer medicine information even notes that a headache may be a sign the medicine is working. The nurse treats it as expected but still assesses it, and does not advise skipping doses to avoid it.
Hypotension follows because venous pooling reduces cardiac output and arterial dilation lowers resistance. The drop is greatest when the client stands, so dizziness, lightheadedness and fainting are possible. Flushing and a reflex rise in heart rate can accompany it. Occasionally severe hypotension is accompanied by paradoxical bradycardia and worsening angina, which needs urgent escalation.
Why PDE5 inhibitors are dangerous with nitrates
Phosphodiesterase type 5 normally breaks down cyclic GMP. Drugs such as sildenafil, tadalafil, vardenafil and avanafil block that enzyme, so cyclic GMP stays high for longer. Riociguat stimulates the same pathway from another direction. When combined with a nitrate, the two effects stack, and vessels can dilate far more than either drug would cause alone.
Prescribing information lists the combination as contraindicated because it can cause severe hypotension, syncope or myocardial ischaemia. In practice, ask every client with chest pain about recent use of erectile dysfunction or pulmonary hypertension medicines before nitroglycerin is given, and report a positive answer to the prescriber. Clients should be taught to tell emergency staff about these drugs too.
Nursing actions before and after each dose
Check blood pressure and heart rate before giving nitroglycerin and follow the prescribed hold parameters. Have the client sit or lie down before a sublingual dose, because standing increases the risk of a sudden drop. Reassess pain, blood pressure and heart rate after each dose, and raise the legs and call for help if the client becomes hypotensive.
Teach clients using sublingual tablets at home to sit down before taking a dose and to call emergency services if chest pain is not relieved as instructed by their plan. Tablets should stay in their original container, tightly closed, away from heat and moisture. Tolerance can develop with continuous exposure, which is why long-acting regimens include a nitrate-free interval.
Apply the mechanism to a hypothetical question
A hypothetical client with chest pain is about to receive sublingual nitroglycerin. Blood pressure is acceptable, but the client mentions taking sildenafil the previous evening. Options include giving the tablet with the client standing, giving it and rechecking blood pressure in 30 minutes, giving half the usual tablet, or withholding it and notifying the prescriber. Withholding and notifying is correct.
The mechanism explains why. Sildenafil keeps cyclic GMP high, and nitroglycerin generates more, so the combined vasodilation can cause profound hypotension and worsen ischaemia. Standing would increase the risk further, and a delayed recheck misses an early drop. Reducing the dose is not a safe workaround, because the interaction itself is the contraindication.
Sources and further reading
DailyMed: Nitrostat (nitroglycerin) sublingual tablets prescribing information. Nitric oxide and guanylate cyclase mechanism, venous predominance, headache, postural hypotension, paradoxical bradycardia, tolerance and contraindication with PDE5 inhibitors and riociguat.
MedlinePlus: Nitroglycerin sublingual. Vessel relaxation lowering heart oxygen need, headache as a sign the drug is working, dizziness on standing, sitting during use and storage.
MSD Manual Professional: Angina pectoris. Venous and coronary vasodilation reducing wall tension and oxygen demand, nitrate tolerance and the need for a nitrate-free interval.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our pharmacology practice questions are the closest set to what this page covers.
One question from the pharmacology set
A client with heart failure is started on furosemide 40 mg PO daily. Which findings should the nurse report to the provider before administering the next dose? Select all that apply.
Rationale
Furosemide is a loop diuretic, so the two things you are watching are potassium and kidney function. A potassium of 2.9 mEq/L is below the 3.5–5.0 reference range and puts the client at risk for dysrhythmia — hold and report. Muscle cramps with palpitations are the clinical face of that same hypokalemia, so they are reported together, not separately. A creatinine that doubles signals the diuresis has outrun renal perfusion. A blood pressure of 132/78 and a 1 kg loss are the expected response to the drug working, not reasons to hold it.
Answer: A, D, E
Common questions
Is a headache after nitroglycerin a reason to stop it?
Not usually. Headache is an expected effect of vasodilation and often lessens with continued use. Report severe or persistent headache, and do not stop prescribed nitrates without advice.
Why should the client sit before a sublingual dose?
Venous pooling and lower blood pressure are greatest when upright. Sitting or lying reduces the chance of dizziness, falls and fainting after the dose.
Which drugs should the nurse ask about before giving nitroglycerin?
PDE5 inhibitors such as sildenafil, tadalafil, vardenafil and avanafil, and riociguat. Combined with nitrates they can cause severe hypotension and syncope.
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