Hydralazine, a blood pressure medication that works by relaxing blood vessel walls, causes a rapid or pounding heartbeat in more than 10% of people who take it. Other common side effects include headache, nausea, diarrhea, and fluid retention. The most serious concern with long-term use is a lupus-like autoimmune reaction that affects roughly 5% to 20% of patients depending on the dose.
How Hydralazine Causes Its Most Common Side Effects
Hydralazine lowers blood pressure by directly relaxing the muscles in artery walls. But because it only works on arteries and not on veins, your body detects the sudden pressure drop and kicks off a compensatory response. Your nervous system releases stress hormones that speed up your heart and increase the force of each beat. This is why a fast or pounding heartbeat is the single most reported side effect, affecting more than one in ten people on the drug.
That same reflex also causes your body to hold onto more sodium and water, because your kidneys sense reduced blood flow and respond by retaining fluid. This can lead to swelling and, over time, can actually work against the blood pressure lowering you’re trying to achieve. For this reason, hydralazine is almost always prescribed alongside a diuretic (to counteract fluid retention) and another medication that slows heart rate (to counteract the racing pulse).
Common Side Effects
Side effects that show up in 1% to 10% of users include headache, nausea, vomiting, diarrhea, and general stomach upset. Dizziness is also frequently reported, especially when standing up quickly, since blood pressure can temporarily drop too low. These effects tend to be most noticeable when you first start taking the drug or when your dose increases, and they often lessen over weeks as your body adjusts.
Drug-Induced Lupus
The most clinically significant risk of hydralazine is a condition called drug-induced lupus, an autoimmune reaction that mimics the symptoms of systemic lupus. Joint pain is the hallmark: it shows up as the first complaint in about 80% of cases, most often in the small joints of the hands and wrists, then elbows, ankles, and knees. Muscle aches can accompany the joint inflammation, and in some patients they become severe enough to be disabling. About 29% of people who develop this reaction also experience chest pain from inflammation around the lungs or heart. Fever, rash, swollen lymph nodes, and persistent fatigue round out the picture.
One important distinction: unlike naturally occurring lupus, the drug-induced version rarely involves the kidneys or brain, which makes it less dangerous overall. In the vast majority of cases, symptoms resolve after stopping hydralazine, though it can take weeks to months for full recovery.
The risk climbs with higher doses and longer exposure. In a study following 281 patients, drug-induced lupus occurred in about 5.4% of those taking 100 mg per day and 10.4% of those on 200 mg per day. After 20 years on doses of 400 mg or more daily, the rate reaches 10% to 20%. A cumulative lifetime dose above 150 grams is a recognized threshold for increased risk. In reported cases, the reaction has appeared anywhere from 6 months to 5 years after starting treatment. Doses above 200 mg per day are generally avoided specifically because of this risk.
Who Is Most Vulnerable
Six well-established risk factors have been identified: higher cumulative dose, female sex, white race, a genetic trait called slow acetylation (which affects how quickly your liver breaks down the drug), and two inherited immune system markers. Slow acetylators process hydralazine more gradually, meaning the drug lingers in the body longer at higher levels, which increases the chance of triggering the autoimmune reaction. People with a family history of autoimmune disease and those with reduced kidney function are also at elevated risk.
Nerve Tingling and Vitamin B6 Depletion
Hydralazine can interfere with vitamin B6 (pyridoxine) metabolism because of its chemical structure. Over time, this depletion can cause peripheral neuropathy: tingling, numbness, or a pins-and-needles sensation, usually in the hands and feet. The symptoms tend to worsen the longer the drug is taken at higher doses. Supplementing with B6 can prevent or reverse this side effect, and some prescribers recommend it routinely for patients on long-term hydralazine therapy.
Blood Cell Abnormalities
The FDA label for hydralazine warns of blood dyscrasias, a group of conditions involving abnormal drops in blood cell counts. These include reduced red blood cells and hemoglobin, low white blood cell counts, dangerously low levels of a specific white blood cell type called granulocytes, and purpura (small bruise-like spots from bleeding under the skin). These reactions are uncommon but serious enough that the FDA recommends complete blood counts before starting treatment and periodically throughout its use. If blood abnormalities develop, the drug is typically stopped.
Monitoring During Long-Term Use
Because of the lupus risk, the FDA recommends blood tests for antinuclear antibodies (a marker of autoimmune activity) before you start hydralazine and at regular intervals afterward, even if you feel fine. If you develop joint pain, unexplained fever, chest pain, or persistent fatigue while on the medication, these tests become especially important. A positive antibody result doesn’t automatically mean you have to stop the drug, but it signals your prescriber to weigh the benefits of continued treatment against the risk of a full autoimmune flare.
Use During Pregnancy
Hydralazine has historically been a go-to option for managing dangerously high blood pressure during pregnancy, particularly in preeclampsia. However, a meta-analysis of clinical trials found it may cause more problems than alternative medications in this setting. Compared with other options, injectable hydralazine was linked to more episodes of maternal blood pressure dropping too low, more abnormal fetal heart rate patterns, and lower newborn health scores at birth. It was also associated with higher rates of emergency cesarean sections and placental abruption. While still used in some clinical situations, the evidence does not support it as the first choice for severe hypertension in pregnancy.
Drug Interactions
Hydralazine interacts with over 400 other medications. Most of these interactions are moderate, meaning they require caution rather than absolute avoidance. The primary concern is additive blood pressure lowering: combining hydralazine with other medications that reduce blood pressure, including common drug classes for heart disease, can cause blood pressure to drop dangerously low. Alcohol amplifies this effect as well. If you’re starting hydralazine and take other medications, a pharmacist can flag the specific combinations that apply to you.