What are the long term complications of fillers?

In the hands of a board-certified cosmetic surgeon or other skilled injector, serious risks and complications of dermal fillers are few and far between. That’s not to say these things don’t sometimes occur, but the best way to minimize complications is to work with someone who knows what they’re doing. Watch out for fraudulent providers who don’t disclose the specific product they are using. If you still have concerns, you might want to go ahead and schedule an in-office consultation with a cosmetic surgeon in your area to help give you peace of mind.

In general hyaluronic acid fillers are very safe and rarely cause long term problems.  However, there are reports of delayed nodules that have developed weeks, or even months later.  Typically the nodules, or areas of swelling, can be treated by injecting a dissolving enzyme or an anti-inflammatory product.  Less often, oral antibiotics may be necessary.
This is why your injector must take a good medical history and employ a sanitary technique for your protection.  Your skin must be prepped with an antibacterial.  In my office, I use a product containing alcohol and a very effective antimicrobial.  I also make sure the patient does not go for dental work 2 weeks before or after injections.  This includes teeth cleaning.
Most experts believe that the nodules are an inflammatory reaction.  Bacterial cultures of these nodules in the treatment area are most commonly negative. Regarding death of skin cells (skin necrosis), yes that is a possibility if a blood vessel is inadvertently blocked off with the injectable product.  Pain and blanching of skin are clues of necrosis, even if delayed.  These reactions must be immediately treated and can resolve without any long-term consequence when treated properly.  
Always be 100 percent sure of what you are getting.  Here in the U.S., the product must be FDA approved and purchased in the United States.  If the price is too good to be true, that should raise a big red flag!  Long-term complications are more frequent with permanent fillers like silicone.   This is why I do not understand how anyone will go for injections at salons, hotel rooms or their local fitness centers.  Yet I hear it all the time, even from the most savvy, educated people.  I cannot stress enough that your injector should be a board certified dermatologist or a board certified plastic surgeon.  Outside of the U.S., look for similar accreditation in your country.