Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes Botox (Onabotulinumtoxin-a). If you are concerned about side effects, discuss the risks and benefits of Botox (Onabotulinumtoxin-a) with your doctor.

The following side effects have been reported by at least 1% of people taking Botox (Onabotulinumtoxin-a). Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

General side effects:

• pain, tenderness, or bruising at the place of injection

When used for blepharospasm:

• drooping of the upper eyelid
• double vision
• dry eye
• irritation or watering of the eye
• not able to close the eye completely
• sensitivity of the eye to light
• swelling of the eyelid

When used for cervical dystonia:

• flu symptoms
• local or general weakness
• nausea
• neck weakness

When used for focal spasticity:

• decreased coordination
• feeling of "pins and needles"
• Fever
• flu symptoms
• headache
• increased sensitivity to touch
• itching
• joint pain
• muscle tightness or weakness
• nausea
• pain in the affected limb (arm or leg)

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• difficult or painful swallowing, dizziness, shortness of breath, or vision changes (when used for cervical dystonia)
• facial paralysis or persistent eye irritation or pain (when used for eye conditions)
• falling (when used for spasticity due to cerebral palsy)
• fever, especially when accompanied by coughing and shortness of breath
• irregular heartbeat
• shortness of breath
• skin rash
• symptoms of a urinary tract infection (e.g. pain when urinating, urinating more often than usual, low back or flank pain)

Stop taking the medication and seek immediate medical attention if any of the following occur:

• breathing problems
• difficulty swallowing
• muscle weakness
• peeling or blistering skin
• seizures
• signs of an allergic reaction (e.g., Hives; difficulty breathing; swelling of the mouth, tongue, or throat)
• signs of a Heart Attack (e.g., chest pain or pressure; shortness of breath; jaw, shoulder, or arm pain; nausea; lightheadedness; sweating)
• speech problems

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are using Botox (Onabotulinumtoxin-a).