At a glance
Botox (botulinum toxin) injection is a well-established treatment for chronic anal fissures that have not healed with topical treatments alone. It works by temporarily paralysing the internal sphincter muscle, reducing the spasm that prevents the fissure from healing.
Like any medical treatment, botox has side effects. This page covers all the commonly reported ones — from mild and expected effects like injection site soreness to the more concerning possibility of temporary continence changes. Understanding what to expect can help you make an informed decision and know what is normal during recovery.
Injection site effects
These are the most immediate and most common side effects. Nearly everyone experiences something at the injection site.
Soreness and bruising
- Mild to moderate soreness at the injection site is common and typically lasts one to three days
- Some people describe a bruised feeling — tenderness when sitting or during bowel movements in the first day or two
- The soreness is usually described as a dull ache, distinct from the sharp fissure pain
- It resolves on its own without specific treatment
Pain during the procedure
- Most botox injections for fissures are done under sedation or general anaesthesia, so people do not feel the injection itself
- Post-procedure pain is generally mild
- People who had the injection under local anaesthesia sometimes describe brief, sharp discomfort during the injection that settles quickly
- Pain that is severe or worsening after the first few days is not expected and should be reported to your doctor
Mild bleeding
- A small amount of bleeding from the injection site is normal
- This is usually minimal — a few spots on toilet paper or a pad
- It typically stops within a day
- Heavy or ongoing bleeding is not expected and should be discussed with your doctor
Continence changes
This is the side effect that concerns people most. It deserves careful, honest discussion.
What happens and why
Botox works by relaxing the internal sphincter muscle. This muscle contributes to the resting tone that helps maintain continence. When the muscle is relaxed by botox, there can be a temporary reduction in the fine control that this muscle provides.
Important context:
- The internal sphincter provides resting tone — it helps keep things closed when you are not actively trying
- The external sphincter (which you control voluntarily) is not affected by the botox injection
- The degree of relaxation varies between people and depends on factors like the dose used and individual anatomy
What people report
Continence changes are not universal. Many people have botox with no noticeable change in bowel control. For those who do experience changes, here is what they describe:
- Reduced gas control — the most commonly reported change. People describe less warning before needing to pass gas, or difficulty holding gas in situations where they normally would. This is usually the mildest form of continence change.
- Minor urgency — some people describe needing to reach a bathroom more quickly than usual. The window between feeling the urge and needing to go becomes shorter.
- Difficulty distinguishing gas from stool — this is described less frequently but is more distressing when it occurs. People describe uncertainty about what is about to happen, leading to anxiety in social situations.
- Minor soiling or leakage — a small number of people report minor soiling, particularly after passing gas. This is usually infrequent and manageable with a pad or liner.
The temporary nature
The single most reassuring aspect of botox side effects, including continence changes, is that they are temporary. Botox wears off. The muscle regains its tone. Any continence changes caused by the botox reverse as the effect fades, typically over two to three months.
This is fundamentally different from surgical procedures like LIS, where the change to the sphincter is permanent. With botox, the body returns to its pre-treatment state.
What people do about it
- Wearing a thin pad or liner during the botox treatment period provides peace of mind
- Being aware of bathroom locations when out — not because emergencies are common, but because the reassurance reduces anxiety
- Maintaining a good diet to keep stool consistency predictable
- Discussing the degree of change with their doctor, particularly if it is affecting daily life
Headache
Some people report headaches after botox injection for a fissure. This is less commonly discussed than with GTN cream, but it does come up.
- Headaches are typically mild and brief
- They may occur in the first day or two after the procedure
- They are not the same mechanism as GTN headaches — botox does not work as a vasodilator
- Most people who report a post-procedure headache describe it as minor and not requiring more than standard pain relief
Incomplete response
While not a side effect in the traditional sense, an incomplete response to botox is an important outcome to understand.
What this looks like
- The fissure pain improves but does not fully resolve
- The sphincter relaxes partially but the fissure does not heal completely during the botox window
- Pain returns as the botox wears off, indicating the fissure did not close
How common is it
Botox does not work for everyone. Success rates vary in the literature, but a significant number of people need either a repeat injection or a different treatment approach. This is not a failure — it is information.
What comes next
- A second botox injection may be recommended
- Your doctor may discuss surgical options such as LIS or fissurectomy
- Conservative care continues regardless of the next treatment step
- The decision about next steps is yours, made with your doctor’s guidance
Allergic reaction
True allergic reactions to botulinum toxin are rare, but they can occur.
Watch for:
- Rash or hives appearing after the procedure
- Swelling, particularly of the face, lips, or throat
- Difficulty breathing
- Any symptoms that feel like an allergic response
If you experience any of these, seek medical care immediately. While uncommon, it is important to be aware of the possibility.
Infection
Infection at the injection site is uncommon but possible with any procedure that breaks the skin.
Signs to watch for:
- Increasing redness, warmth, or swelling around the injection area
- Discharge or pus
- Fever
- Pain that is worsening after the first few days rather than improving
If you notice any signs of infection, contact your doctor promptly.
What is normal versus what needs attention
This is the question most people have in the days after the procedure. A general guide based on what people report:
Usually normal
- Mild soreness for one to three days
- A small amount of bleeding on the first day
- Minor changes in gas control
- The fissure still being painful for the first week or two (the botox has not reached full effect yet)
- Feeling anxious about whether it is working
Worth mentioning to your doctor
- Pain that is not improving after two weeks
- Continence changes that are affecting your daily activities
- Bleeding that is increasing rather than decreasing
- No improvement in fissure symptoms after two to three weeks
Seek care promptly
- Severe pain at the injection site
- Fever
- Signs of infection
- Loss of bowel control that is significantly affecting daily life
- Any signs of an allergic reaction
Making the decision
Understanding side effects is an important part of deciding whether botox is right for you. A balanced perspective from what people report:
- Most side effects are mild and temporary
- The most concerning possible side effect — continence change — is also temporary with botox
- The majority of people who have botox for a fissure describe the side effects as manageable
- The side effects need to be weighed against the ongoing impact of the untreated fissure
- Your doctor can help you understand the specific risks based on your individual situation
Questions to ask your doctor
- What side effects do you see most commonly in your patients?
- What dose of botox do you plan to use, and how does that relate to the risk of continence changes?
- What should I do if I notice changes in bowel control?
- When should I contact you after the procedure versus waiting for my follow-up?
- If botox does not fully work, what are the next options?
If you experience loss of bowel control affecting daily life, severe pain at the injection site, fever or signs of infection, or an allergic reaction, seek medical care.