Guanfacine Side Effects: What to Expect and When to Call Your Doctor

Guanfacine is generally well-tolerated, but like all medications it can cause side effects. Most side effects are mild and tend to improve as your body adjusts, but some require medical attention. Here's what to expect and how to tell the difference.

Most Common Side Effects of Guanfacine

Clinical trials have identified the following as the most frequently reported side effects of Guanfacine ER (Intuniv):

Somnolence / Sedation (sleepiness): The most common side effect, affecting up to 35% of patients in clinical studies. Usually most prominent in the first few weeks and may improve over time. Taking Guanfacine in the evening can reduce daytime drowsiness.

Fatigue: Feeling more tired than usual, particularly during dose adjustment. Common in the first 2–4 weeks.

Headache: Reported in approximately 24% of patients in trials. Usually mild and resolves without intervention.

Abdominal pain / stomach upset: Reported frequently, especially in pediatric patients. Usually mild; taking with a small amount of food may help (but avoid high-fat meals with ER formulation).

Dry mouth: A common alpha-2 agonist class effect. Staying well-hydrated helps; some patients use sugar-free gum or lozenges.

Constipation: Can occur; increased fluid and fiber intake typically helps.

Low blood pressure (hypotension): Guanfacine lowers blood pressure as part of its mechanism. Mild dizziness upon standing (orthostatic hypotension) can occur, particularly when starting or increasing the dose.

Decreased appetite: Some patients experience mild appetite reduction. Less pronounced than with stimulant medications.

Bradycardia (slow heart rate): Guanfacine slows heart rate as part of its mechanism. Monitor your resting pulse, especially when starting treatment.

When Do Side Effects Usually Start and Improve?

Most side effects of Guanfacine peak during the first 1–2 weeks of treatment and during dose increases. For many patients, sedation and fatigue improve significantly after 2–4 weeks on a stable dose. If side effects are intolerable or persist after several weeks, talk to your prescriber — a dose adjustment or dosing time change may help.

Serious Side Effects: When to Call Your Doctor Immediately

These side effects are less common but require prompt medical attention:

Fainting (syncope): If you faint or feel like you're going to faint, stop activity immediately, sit or lie down, and contact your doctor.

Very slow or irregular heartbeat: If your pulse is consistently below 50 beats per minute, or if you notice an irregular rhythm, contact your provider. In rare cases, Guanfacine can cause heart rhythm changes that may lead to serious complications.

Allergic reaction: Hives, difficulty breathing, or swelling of face, lips, tongue, or throat. Call 911 or go to the nearest emergency room.

Severe skin rash or skin peeling: Skin rash with exfoliation has been reported. Contact your provider immediately.

The Most Important Warning: Never Stop Guanfacine Abruptly

This is arguably the most critical safety point with Guanfacine. If you suddenly stop taking it — especially at higher doses — you may experience rebound hypertension: a rapid and potentially dangerous spike in blood pressure. You might also feel anxious, agitated, and have a racing heart rate. If you need to stop Guanfacine for any reason, your prescriber will give you a gradual tapering schedule to reduce these risks.

Tips for Managing Common Side Effects

Sedation: Take Guanfacine at bedtime. Avoid alcohol and other sedating medications. Avoid driving until you know how it affects you.

Dizziness/low blood pressure: Rise slowly from sitting or lying positions. Stay well-hydrated. Avoid becoming overheated or dehydrated.

Dry mouth: Sip water throughout the day; use sugar-free gum or candy to stimulate saliva.

Constipation: Increase fiber and fluid intake. Talk to your doctor if it becomes severe.

Side Effects in Children vs. Adults

Guanfacine ER is FDA-approved for ADHD in children ages 6–17. Pediatric patients tend to experience more pronounced sedation than adults. For children, it's important to monitor school performance, especially at the start of treatment, as drowsiness can affect learning. Weight-based dosing is used in children, so any significant weight changes should be reported to the prescriber.

For information on medications that can worsen Guanfacine side effects or reduce its effectiveness, see our guide: Guanfacine drug interactions: what to avoid and what to tell your doctor.