The main reason for the dropping of the eyelid is the placement of the Botox being too close to the orbit margin. If the injection point is too close to the edge of the orbit, the medication will drift into the upper eyelid and relax the muscle. The degree of dropping is more if the concentration is higher. If this occurs, the a minor drop can be corrected with the use of eyedrops. Otherwise, you'll have to wait for the Botox to wear off.The number of units of Botox used is less important than the concentration of the Botox solution. Some injectors use high concentrations which means small volumes. Other injectors prefer to use low concentrations which means a larger amount of fluid injected into the brow. So, if a large amount of fluid is injected into the brow, this fluid can spread through the muscles and perhaps move into the upper eyelid. Also, the number of units of Botox required depends on the degree of muscle strength. The stronger the muscle, the more units required to get the result.Experienced injectors follow a rule where you can always add more Botox, but you cannot take it away. Having injections done by someone with lots of experience may not guarantee that there will be no eyelid drop, but it can ensure that it is much less likely that it will occur.