We know Botulim Toxim is obtained from bacteria which is a protein that is purified and injected in miniscule amounts. To give you an example, a whole bottle of Botulim Toxim contains about 35-40 nanograms which is miniscule amounts.
As with any medicine, taken in the right amounts it is safe but if you take too much then it can cause damage. However, we know that if you use Botox repeatedly it can cause antibodies and that can prevent botox from working.
But how long does it last? It depends on many factors, some patients muscles wake up quicker. Men have thicker muscles so they need botox. It depends on how much you use the muscles but normally a jab lasts between 3-6 months. Sometimes more, sometimes less.