Will I Get Permanent Side Effects if I Try Botox Just Once?

Botox used only a few times is unlikely to cause permanent problems - outside of complications (it is possible - though extremely unusual - to have a hematoma from a botox injection that may leave some scar tissue or contour deformity).
Botox itself usually does not have an effect that lasts beyond six months.
There is some discussion that being treated for years with Botox can cause some atrophy (weakening and shrinking) of the muscles it is used to treat but this is usually a subtle change after long term usage.
I hope this helps.

Botox has minimal and very short-term side effects frim having an injection: swelling, redness, soreness and maybe some bruising. These should resolve within days or up to two weeks, maximum. There are no long-term side effects with Botox, as it is a temporary procedure. You can try Botox once. If you don't like it, there are no permanent effects. Botox only lasts about three to four months. If you want to continue your good results, you'll need to continue your Botox treatments. Please see a board-certified dermatologist to get your Botox so that you can be sure you are being treated optimally.

Thanks for the question many people have. If you do Botox, 5 months later you will return back to the way you were before the Botox was injected. It is not something that has to be continued unless you want to maintain the effects. 

Botox is a very safe drug. Millions have used the treatment. It is the most popular cosmetic treatment in the world. That being said, there are always risks, (albeit very small), of any treatment.  Usually people like the result and then will keep up with the treatments.  It is important to choose your injector wisely. Ask questions about training and experience. Have realistic expectations, and take the time to tell your injector what your goals are.

I can reassure you
that if you try Botox once, it does not mean that you will have permanent
changes, and you do not have to continue with Botox treatments in the future.
The results of Botox wear off within 3-4 months, so that if you were unhappy in
any way with your treatment, your appearance will return to your baseline by
that time. However, most people love the results from their Botox treatment,
and want to continue with their treatments!

There are no substantial long term complications from botox, especially if you only try it once. Once the botox stops working, you should return to your normal appearance. If you use it long term, there may be some long term benefits with regard to lack of wrinkling in certain muscles.