I did Botox 20 days ago for crows feet and it only worked on one side. How long will it take for touch up to kick in?

Botox takes 3-5 days to start working and about 10 days for full effect.  This is the same for the touch up.  You may be needing more units and not just a touch up, since the FDA recommended dose for the crows feet is 12 units in each side.

Hello Crooked smile 29,As a rule of thumb, you will begin to notice Botox working in about 3 days, reaching maximum effect in about 7-10 days.  This is true whether you had a touch up or the original treatment.  I personally like to start around 10 units of Botox for each crow's feet and that works the first time in 90% of my patients so with a total of 12 units you should notice a nice result.  I hope this helps and good luck.

It is not common for one side crow's feet not to respond to Botox and if 10 units did nothing, I'm not sure if 6 more will suddenly relax the muscle but it generally take 72 hours to start seeing results and may take 7-10 days to see the final result of that treatment.