Can getting Botox only once a year prevent wrinkles developing?

I would suggest doing your Botox every 4 months if your goal is to prevent wrinkles.  It is still worth it if you only do it once a year.  It will help.

Thank you very much for your question. Yes, Botox injections can halt the development of fine lines and wrinkles. By temporarily weakening targeted muscles, the tissues will relax, smoothing lines and preventing deeper, dynamic wrinkles (caused by repetitive expressions) from forming. On average, the results from Botox will last from three to four months, at which time a repeat session is recommended. Yearly Botox injections will help in the prevention of wrinkles, although not as effectively as a maintenance treatment every three to four months. 

Botox is a great way to keep wrinkles from becoming permanently etched in the skin and it's best to get repeat treatments every 3-4 months. At best, I recommend coming twice per year for treatment but yearly treatments will certainly be better than never getting Botox. I wouldn't consider it a waste but would recommend at least twice per year if possible. Thanks.