Will 1/2 a syringe be enough for maintaining a previous injection along my smile lines?

It's possible, but it's difficult to answer effectively without pictures. I recommend following up with your injectors. I hope this helps. 

Hello and thanks for your question. Maintaining previous injections typically requires at least 1 syringe in order to provide a nice result. Please keep in mind that 1 syringe is a very small amount and would provide a modest result for a small area such as tear trough (would start with 1/2 syringe per side). Nasol folds and marionette lines require more than 1/2 syringe in most cases. All the best!!

Thank you for your question. 
Glad you are satisfied with your filler from one year ago. The syringes are not sold in 1/2 syringes.   Usually if one has volume loss in marionette lines, there is volume loss in the cheek area and  it is usually very easy to use one full syringe.  Often times there may be volume loss in the temple region or nasolabial folds.  Maintainance is great and using the full syringe should warrant nice results.
Please consult your surgeon injector or a local board certified facial plastic surgeon for more information and a comprehensive consultation 

Without a photo or in person consultation it is impossible to suggest an answer. However, if the line is no longer deep then a small amount of filler may be all that is needed. Juvederm comes by the syringe and not a half syringe. Please consult an expert in fillers for the best cosmetic results. Best, Dr. Green

Without seeing you in person it would be hard to say.  Usually syringes are not purchased in 1/2's.  ...

In order to be able to provide proper answers in tour case you need to submit a photograph of yourself or be evaluated in person. Good luck from New York City

It all depends on the depth and extend of your lines. Your description suggests rather minor problem so 1/2 syringe should do the job. If your lines are quite superficial only a small amount of Juvederm should be injected to avoid bluish discoloration or Tyndall effect. If small amount is used than the filler's particles are few and far between to matter; if too much is injected the density of the molecules will be high enough to bounce back(reflect) the blue color light wavelength and will appear blue to our eyes. Good luck!

Thank you for your excellent question.  It is difficult to comment on your case without seeing photographs.  Juvederm can last for up to 1 year, longer in some patients, shorter in others, depending on individual metabolism.  It is certainly possible that half of a syringe would be enough for maintenance.  How many syringes did you receive initially?  After optimal treatment, I recommend that my patients consider a touchup at 8 months.  Please be sure to see your injector for an in-person reevaluation in order to determine the best treatment plan.  Hope this helps and best wishes!