What are possible side effects of Silikon 1000 in the lips?

Silikon-1000 is a permanent filler, so your results, good or bad, are permanent. You must find a board-certified dermatologist or facial plastic surgeon experienced with the micro-droplet technique. You should also have a discussion with your doctor about the FDA's position with respect to using Silikon-1000 in an off-label fashion to enhance your lips. Lastly, you should see many before and after photos demonstrating beautiful results before you pick your doctor and proceed.
In my hands, complications with Silikon-1000 are infrequent. Around 3% of patients will develop a nodule or bump at the site of injection. These typically respond well to dilute triamcinolone injections; rarely they need to be excised. Also, several treatments may be necessary to obtain a desired result.
I've attached a link to my Lip Enhancement photos for your perusal.
I hope this is helpful for you.

In the right hands, liquid injectable silicone (Silikon-1000) is the ideal filler for lip augmentation as it is very precise and gives the most natural-looking results. It is of course permanent, and therefore, you should only consider being treated by a board-certified dermatologist or facial plastic surgeon with experience and expertise in the use of liquid injectable silicone. That being said, as with any filler treatment, side effects include bruising and swelling, which typically resolve within 5-7 days. Very rarely, and not yet in my hands, one can develop a small nodule or bump at the site of injection. However, this is usually related to the injection technique, meaning the silicone was injected too superficially or too much was put in with the injections.