Can Botox on Crows Feet Cause Muscle Paralysis in Other Areas of my Face?

It is possible that the injections you had for your crow's feet migrated a bit to the muscles in the upper face that control your smile. There are several issues that can affect this.

We usually do not need 20 units for the crows feet. It is typically about 15 for our patients so it's possible a bit more than was necessary was injected.
It could be that the placement of the product was not exactly where it should be and moved to affect other muscles in the area.
Finally, it could be that the migration is in part caused by the fact that the product is overdiluted. The more saline is used with the Botox, the higher the probability that it will migrate to other areas of the face. Different MDs dilute with different amounts of saline and it's worth asking if his/her dilution is higher than is recommended by Allergan (the manufacturer).

First of all, you should only be injected by an experienced Board Certified Plastic Surgeon or Dermatologist.  Second of all, injections for the crow's feet can be very effective, but your injector needs to be very careful not to inject too deep or with too much volume to prevent seeping of the Botox to the levator muscle for the lip.  If this happens, it can affect your smile.  The effect will go away, since Botox is never permanent, but may last for several weeks to months.  

Botox works well but it really is a toxin, and can not be made to wear off extra quick, so too much Botox or placement in the wrong area can really be a problem. Do not take injection lightly, even though it can seem easy when done by an experienced, board-certified dermatologist or plastic surgeon.

It is possible that too much Botox was placed, or that it was placed in the wrong area, if you feel heaviness in your smile and in your cheeks when you were just trying to soften crow's feet. The good news is that it will wear off in 3 months, and you can either correct this result with your next treamment by discussing with the doctor, or find someone else you might be more comfortable with.

 

After injection of Botox to the lateral eye area, a small amount of spread is expected. This is the approximately 1 inch in diameter. It is very unusual for Botox to spread beyond this area and affect other muscles of the face. If you are concerned, we visit with your surgeon can describe your concerns. It is important to determine whether this effect is from the Botox or from a mother neurological condition.

The peak effect of Botox is 1-2 weeks. 20 units of Botox is a typical dose for the crows feet, but it may be that you require less. Botox can diffuse 3 cm or greater from the point of injection so a low injection of the crows feet could very well have spread to the muscles that affect your smile, more on one side than the other.
I suggest you be evaluated by the practitioner who performed your Botox injections to rule out other conditions unrelated to Botox such as Bell's Palsy (facial nerve paralysis). If your asymmetric smile is simply related to the Botox, improvement should occur within 1-2 months with full resolution by 3-4 months.
Good Luck!