DEAR DR. FOX: I have a 13-week-old puppy, and I used Activyl on him for fleas. Until your communication, I was not aware that indoxacarb -- the highly toxic, hazardous insecticide in Activyl -- was approved for companion animal use. According to one report from the European Medicines Agency, “The most common side effects seen with Activyl Tick Plus (which may affect up to 1 in 10 animals) are transitory scratching, erythema (reddening of the skin) and hair loss at the application site. ... Activyl Tick Plus must not be used in cats as adverse reactions and even death can occur. The volume of mail received prohibits personal replies, but questions and comments of general interest will be discussed in future columns.