At a glance
GTN (glyceryl trinitrate) ointment is one of the most commonly prescribed topical treatments for anal fissures. It works by relaxing the internal sphincter muscle, improving blood flow to the fissure, and creating conditions for healing. Headaches are a common side effect but often improve over time.
How it works
The internal anal sphincter is a ring of smooth muscle that stays partially contracted at all times. In people with fissures, this muscle tends to go into spasm — clamping down around the fissure and reducing blood flow to the area. Reduced blood flow means slower healing, which means the fissure persists, which means more spasm. It is a cycle.
GTN breaks this cycle. When applied to the anal margin, it releases nitric oxide, which:
- Relaxes the sphincter muscle — reducing the spasm
- Increases blood flow — allowing nutrients and oxygen to reach the fissure
- Reduces resting anal pressure — creating a lower-tension environment for healing
How to apply it
GTN is applied to the outside of the anal canal — specifically the anal margin, not deeply inside.
A typical routine:
- Clean the area gently (a sitz bath or gentle wash)
- Squeeze a small amount of ointment onto a gloved fingertip — typically described as pea-sized or the length of a match head
- Apply gently to the anal margin — around the opening, not inserted inside
- Wash hands thoroughly afterwards
- Repeat as prescribed — typically twice daily (morning and evening)
Important notes
- Use a small amount. More is not better. Excess GTN increases side effects without improving effectiveness.
- Apply to the margin. The ointment needs to reach the sphincter muscle, which is at the entrance to the anal canal, not deep inside.
- Be consistent. GTN works best when used regularly at the prescribed frequency for the full course.
- Wear a finger cot or glove. GTN can be absorbed through the skin of your fingers and cause headaches from finger contact alone.
Side effects
Headaches
The most common and most discussed side effect. GTN dilates blood vessels throughout the body, including in the head. People describe headaches ranging from mild to severe, particularly in the first few days.
Strategies that help:
- Take a standard painkiller (paracetamol) 30 minutes before application
- Use the minimum effective amount
- Apply while lying down
- The headaches often become milder after the first week as the body adjusts
Other side effects
- Dizziness or light-headedness — from the blood pressure-lowering effect. Stand up slowly.
- Flushing — a feeling of warmth, particularly in the face
- Stinging on application — usually brief
When to stop and contact your doctor
- Severe headaches that do not respond to painkillers
- Fainting or significant dizziness
- Rash or severe irritation at the application site
- Any reaction that concerns you
What to expect over time
- Week 1 to 2: Side effects may be most noticeable. Fissure pain may begin to improve.
- Week 2 to 4: Gradual reduction in pain during and after bowel movements. Headaches often lessen.
- Week 4 to 8: Continued healing. Some people are significantly better; others see slower progress.
- After the course: If healed, the fissure may not recur. If not healed, the next step is typically a conversation about alternatives.
GTN alongside other measures
GTN works best as part of a broader approach:
- Stool softening — fibre, hydration, stool softeners
- Sitz baths — especially after bowel movements and before GTN application
- Dietary management — avoiding constipation triggers
- Reduced straining — better toilet posture, not rushing bowel movements
GTN addresses the muscle spasm. These measures address the mechanical factors that caused the fissure and keep re-injuring it.