Thank you for your question regarding your 6 month old girl Lady Sif who managed to ingest up to 8 x 4mg Tizandine tablets about an hour ago. As you probably know, Tizandine is a muscle relaxant, which can cause a number of very worrying symptoms in dogs, particular an ingestion like this in your girl. Symptoms of this type of ingestion in your girl could include anything from depression, disorientation and weakness, to vocalization, seizures, coma and even the risk of death.
You have done exactly the right thing here to induce vomiting. If your girl hasn't vomited yet, then do try again. There are several ways to induce vomiting, however 3% hydrogen peroxide is the most effective as it sounds like you have used already. To double check you induced vomiting the right way, you can read more about this online here:
If she has vomited, but you haven't seen all 8 of the tablets in the vomitus, then you need to get her down to your local ER vet. Likewise, if she doesn't vomit after a second lot of hydrogen peroxide, then definitely get her seen. Because of the severe symptoms that can occur with an ingestion of this nature, it isn't worth taking any risks. Here your vet can give your girl intravenous fluids, activated charcoal (to absorb any more of the drug in her intestines), and keep a close eye on her mentation and blood pressure through the night.
Given the large ingestion of this drug, as well as the potential serious consequences of her ingestion even just 1 x 4mg tablet, I would recommend you play it safe and take her down to your local ER vet regardless of whether she has vomited already or not. It is always better to play it safe with this type of ingestion. I hope all of the above makes sense? If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
PS: If you have additional questions after you rate the question, you are welcome to request me for additional conversations if I am on-line or by beginning your question "Dr. E..." or "Pet-doc..." and others will leave the questions for me.