Some patients may have a severe allergic reaction to Botox. All over body swelling, difficulty swallowing and/or difficulty breathing are the most likely symptoms that an allergic reaction is occurring. Patients who experience any or all of these symptoms after a Botox® injection should have a person nearby call 911 immediately. Patients should not wait to see if the symptoms resolve or try to take themselves to the hospital.

In some cases, Botox® may prevent normally functioning cells that were not causing undesired symptoms (such as muscle spasms, wrinkles or sweating) from receiving chemical messages. As a result, the muscles or the glands other than those causing undesired symptoms may be paralyzed. Therefore, the body part that received the injection may appear abnormal because it cannot function as usual. These effects are said to wear off between six weeks and six months after the Botox® injection was administered.

Patients who are taking medications, drugs, herbs, or supplements containing atropine and/or pralidoxime should not receive Botox® injections. These chemicals increase the toxicity of Clostridium botulinum and may lead to serious and life threatening side effects.

Patients who have been diagnosed with Lambert-Eaton syndrome, myasthenia gravis or Amyotrophic Lateral Sclerosis (ALS, also known as Lou Gehrigs Disease) are at an increased risk of serious and undesired side effects from Botox®.

Overactive nerve or gland cells: When Botox® is injected underneath the skin, it prevents the nearby cells from releasing a chemical called acetylcholine. When acetylcholine is not released, nerve impulses that might normally cause the patient's gland or muscle cells to behave in a way that causes undesired symptoms, such as excessive sweating or muscle spasms, are blocked. For instance, if Botox® blocks sweat glands from receiving a message to perspire, then sweat glands will not produce sweat. Similarly, if Botox® blocks signals to muscles, then contraction and movement will be prevented, which may lead to a decrease in wrinkling.

Blepharospasm (eyelid spasms): Blepharospasm is condition where the eyes blink rapidly and uncontrollably. These eye movements are caused by twitching in many small muscles in the eyes. This movement causes difficulty in sight, because the eyes are closed much more often than in patients without the condition. As a result, patients may experience difficulty in everyday tasks, such as driving or reading. Botox® may help these patients by blocking the message from the brain that tells the eye muscles to move. When these muscles in the eyes do not move as often, the eyes stay open longer, and often enough for the patient to see with much less difficulty.

Cervical dysplasia (neck spasms): For people with cervical dysplasia (CD), the muscles in and attacheding to the spine in the neck tighten or spasm in unexpected ways. This unexpected movement may feel painful and make it difficult to hold the neck in one position. As a result, the neck muscles may force the head to move in ways that can be awkward and undesired. Keeping the head in one position in order to conduct daily activities such as reading or driving may become difficult. Botox® causes some paralysis in these muscles, so that they can not jerk the head and neck around. The pain caused by unpredictable neck and head movements may decrease, because the muscles causing the sensation are no longer able to move.

Glabellar lines (eye wrinkles): Glabellar lines refer to creases between the eyes. These creases do not cause any medical difficulties, but some people consider them cosmetically unattractive and may wish to have the skin in this area appear smoother. Botox® causes the muscles in this area to relax, and the wrinkles become less noticeable.

Axillary hyperhidrosis (underarm sweating): Some people may experience uncontrollable and excessive sweating under their arms. Although deodorants, antiperspirants and other medications are available, these treatments sometimes do not provide enough relief from the symptoms for some patients. Excessive underarm sweating can stain clothes, cause undesired body odor and may appear unattractive to the patient. Botox® injections in the armpits block the chemical signals that cause the glands to produce more sweat than they should. When the glands in the armpits do not receive this signal, they do not produce sweat, and the patient may experience relief from symptoms.

Strabismus (crossed eyes): Strabismus is a disorder that causes one eye to be constantly or intermittently misaligned compared to the other when focusing. Uneven tightening around some of the muscles that control the eyeballs causes this condition. Botox® is sometimes used along with certain surgeries to weaken the overactive muscles causing the crossed eyes. This may then help the eyeballs return to their normal position.

Restless legs syndrome (RLS): A 2006 observational case series by Rotenberg et al. evaluated the effect of Botox ® on the alleviation of symptoms in three patients with refractory restless legs syndrome (RLS). The three patients in this study experienced relief in the three measures of efficacy: alleviation of symptoms, reduction of medication use and/or reduced daytime sleepiness.