Do Lip Injections Cause Scar Tissue to Form?

Lip augmentation is usually performed by injecting material, such as collagen, hyaluronic acid, or fat. Cosmetic surgery may also be performed to plump or augment the lips.
All cosmetic treatments, surgical and non-surgical, have the risk of adverse outcomes such as scarring. Bumps in the lips after lip injections may be from scar, but also occur from the injected material itself. Most injectable fillers (Juvederm, Restylane, etc) are temporary, and those bumps are gone by several months.
All lip augmentation treatments will temporary make the lips feel firmer in the short term. This brief firmness usually lasts a few hours, to a couple days, depending on the exact treatment and your body's natural healing ability.

Any cosmetic procedure, even Botox has the potential to cause scar tissue to form. Sometimes if your skin (whether it is the lips or other locations) has a significant inflammation in response to filler material (uncommon), scar tissue can result from the actual inflammation. I consider hyaluronic acid fillers (e.g. Juvederm and Restylane) to be the safest and most appropriate for lip enhancement because they are the most forgiving, produce a natural, soft feel and appearance, and any lumps that form can usually be massaged out.
Hope this helps.