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If applicable, use a tranquilizer from a previous seizure. Use a rectal suppository. Administer the tranquilizer as quickly as possible.
If your pet has had seizures before, your veterinarian may have prescribed tubes of rectal Diazepam for use during or after a fit. Diazepam is a tranquilizer and good at calming brain activity sufficiently to bring some dogs out of a fit.  Because your dog cannot swallow oral medications during a seizure, a convenient method of delivery is rectal administration. Diazepam is well absorbed across the rectal mucosa and swiftly enters the blood stream.  A 20 kg dog requires one 10 mg rectal syringe of diazepam, as soon after the seizure starts if possible. The dose can be repeated three times during a 24 hour period. To instill the rectal diazepam, remove the cap from the dosing syringe and squeeze a blob of the Diazepam onto the nozzle tip in order to lubricate it. Here's how to gently and safely administer it to your dog:  Gently hold your dog's tail away from the anus so that you can see the rectal opening. Place the nozzle against the central opening and with a gentle twisting motion push the neck of the treatment syringe into the rectum up to the depth of the syringes shoulders. With the suppository firmly seated in the rectal lumen, squeeze the tube to empty the contents into the rectum. Withdrawal the nozzle and throw the spent suppository away. The sooner you give your dog the diazepam, the sooner his symptoms will likely fade. However, do not compromise your sense of calm in your haste. Though time is important, it's more important that you do not aggravate your dog any further. You can also give your dog the medication after a fit to help him stay calm and prevent future episodes, especially if he is prone to cluster seizures.