The U.S. Food and Drug Administration issued an alert for the public and professionals (September 2018). The products mentioned in the warning are very popular flea and tick preventatives. Many veterinarians I spoke with regularly prescribe these products for their patients and have not been seeing cats or dogs with symptoms such as the ones described in the alert. The FDA considers these products safe to stay on the market. If, however, you have concerns about the continued use of these products, you should contact your veterinarian to discuss the issue.
Here is more information on the topic:
The Food and Drug Administration has issued an alert advising pet owners of the potential for adverse reactions (muscle tremors, loss of muscle control and seizures) in dogs and cats when treated with drugs in the Isoxazoline class. The FDA continues to consider these products to be safe and effective for dogs and cats but is providing this information so that pet owners and veterinarians can consider it when choosing flea and tick products for their pets.
The drugs under this alert include commonly prescribed flea and tick preventatives such as Bravecto, Credelio, Nexgard, and Simparica. The FDA considers these products safe.
Additional information can be found on the FDA website.