If I were to have an allergic reaction to Botox, how long does it take to show up?

Even though Botox is used to treat migraine headaches a small subset of patients do develop headaches after treatment with Botox. This is not an allergy. Allergic reactions to Botox present as a rash or hive type appearance and occur shortly after injection. The likelihood of having an allergic reaction if you have already had Botox before makes it even more rare. Some people do have allergies to the preservatives in the saline which is used to dilute the Botox.

Since you have had Botox before, the likelihood of developing an allergic reaction is extremely low. Headache can be a side effect of Botox. Best of Luck.   

An allergic reaction to Botox is extremely rare and if present would likely manifest in the first 24 hours after injection.  Since you've had it 5 times before, it is highly unlikely that you would develop an allergy to the product.  Headache is a symptom that is fairly dose dependent.  In my practice, I routinely tell patients that receive greater than 50 units in one setting that they may experience a headache.  Botox does not reach your brain.  Your skull is very thick and the product is placed in the muscles of the face near the skin surface anyway.  Don't panic!

Allergic
reactions to Botox are rare, and they would usually show up within 24-48 hours
of the injection. It is unfortunate that
Botox injections cause headaches for you.
This is a relatively uncommon side effect, but it is usually only
temporary. Less than 1% of patients may
develop severe, life-altering headaches that can persist for 2-4 weeks. The cause of these headaches is unknown. The headaches are not caused by Botox
entering the brain. Botox injected
locally to the face does not travel through the skull and into the brain. I recommend that you discuss your concerns about
Botox injections with your board-certified facial plastic surgeon, who can
provide you with further details about the safety of Botox.

Cosmetic Botox injections do not affect the brain. The medication does not travel outside a dime sized area of skin. Allergies either look like hives, which appear immediately, or like eczema, which can take a week to appear. Headaches are not a sign of allergy.

We had a couple patients in the past have a seemingly true allergic reaction to Botox, and one person to all three Botox, Dysport, and Xeomin.  They developed localized itchy, hive-like swellings around each injection site that resolved with ice and oral antihistamines.  Both patients developed the reaction within ten minutes of injection while still at the office.  Both patients had a history of previous Botox treatment, one patient for many years even.  This implies that the patients needed to have had previous sensitization with a past treatment.  

Allergies to Botox are incredibly rare. I have not had a patient (knock on wood) every have an allergy to Botox in over 20 years of injecting it. I've had patients with allergies to a topical numbing cream applied prior to Botox, but even that is less than 1 in thousands of people. Botox doesn't travel to the brain, and your headache is a pretty common side effect. The panic that you are feeling now isn't helping relieve the headache so try to calm down and relax. You aren't having an allergic reaction to Botox. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."

Botox injections around the surface of your face or body will not reach your brain. So, no need to worry about any potential central nervous system (brain) issues. Thanks for sharing your concerns. Best wishes.