Caro
Moderator
As mentioned, steer well clear of products containing Imidacloprid. That includes Seresto and Advantage/Advantix. The American Shetland Sheepdog Association research found Shelties that used these products are 9.3x more likely to get gallbladder mucoceles. It's the only flea/tick product that they found cause they problem, they don't know why yet.
https://www.americanshetlandsheepdogassociation.org/gallbladder-mucolceles-2/
https://www.americanshetlandsheepdogassociation.org/gallbladder-mucolceles-2/