BC powder contains not only asprin but also salicylamide and caffeine. The latter two ingredients are not recommended to be given to a dog. Asprin alone can increase the risk of bleeding adding salicylamide increases this affect and is not recommended. Caffeine can cause severe reactions including hyperactivity, fevers, and pacing actually causing the pet to be more uncomfortable then the initial reason for giving the medication in the first place. The other problem is trying to get an accurate dose according to your dogs weight. BC powder contains 625mg of asprin and depending on how much your dog weighs this could be too much or too little. I will agree that giving just plain asprin tonight is recommended. You can dose your dog at 5mg per pound up to three times daily with food. This is only allowed as long as your dog is not currently on anti-inflammatories and does not have any known bleeding disorders.
Please let me know if further assistance is needed.