The method for using deworming medication for cats, both internally and externally, is to simply apply the medication to the cat's skin.
The method of using feline deworming medication both internally and externally involves applying the medication to the cat's skin. The medication is absorbed into the bloodstream through the skin, and a portion enters the digestive tract and bloodstream through the mouth when the cat grooms its fur. Pharmacokinetic studies have shown that selamectin in plasma exhibits sustained absorption after topical application. 
Precautions
1. Understand the dosage and administration: For external use, the dosage of selamectin is 6 mg per 1 kg of cat. Part the fur on the back of the cat's neck and in front of the shoulder blades to expose the skin. Squeeze the medication onto one or more areas of skin. Do not massage the application site. Do not apply the medication to broken skin.
2. Possible adverse reactions: Some cats may experience inflammatory or non-inflammatory hair loss due to excessive grooming at or around the application site. Some cats may also exhibit vomiting, drooling, or diarrhea that may be unrelated to the treatment. Cat owners should observe these reactions closely, and if they persist for an extended period, it is necessary to consult a veterinarian.
3. To avoid affecting the efficacy of the medication, do not apply the medication to a cat with damp fur; bathing the cat 2 hours after applying the medication will not reduce the efficacy of the medication.
4. For combined internal and external parasite medications, use whichever is more frequently needed. For free-roaming cats, it is recommended to deworm them once a month, while for indoor cats, it is recommended to deworm them 2-4 times a year.
