So, Zeuterin (or Esterisol) is a relatively new means of neutering male dogs in the United States (much longer outside) and there hasn't been a lot of long-term information yet established about it. However, there are studies and the information has been summarized in a comparison paper by Drs. Levy and Crawford. Some possible side effects from that information include:
Short-term effects such as swelling were reported inconsistently.
Necrotizing of the tissue (tissue death) in the area, about 4%, was seen in large dogs at the age limit for the activity.
The Alliance for Contraception in Cats and Dogs has a position paper on the subject. In summary, possible complications:
Testicular swelling and pain (short term).
Vomiting.
Necrosis or ulceration of the injection site.
Additional, localized, side effects as described in documentation.
Of note, and seems to be backed up by the comparison study initially linked (disclosure: found on the same site), the treatment has been used extensively for more than a decade with no long term side effects being reported. Given average lifespans of dogs, I think it's a long enough window to be called multi-generational.
