Can I get Botox before the last Botox treatment wears off?

Yes, it is possible to add Botox before the effect from last treatment completely wears off. The dosing will be based on the muscle strength at the time of appointment. 

Many patients of mine often travel or have social obligations and aren't available to come in when their Botox will wear off and it is reasonable to inject more Botox in the same areas. Usually at three months most have worn off and it is a good idea to have more before it all has worn off to minimize the reformation of lines. Certainly if it is only two months or less since the last treatment and I see no recurrent muscular activity and they are able to return to my practice in 6 weeks I would encourage them not to waste their money and wait for that time to undergo more Botox treatment.The information provided in Dr. Shelton's answer is for educational purposes only and is not intended to constitute medical advice.  The information provided should not be relied upon as a substitute for consultations with a qualified health professional who may be familiar with your individual medical needs.