Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Summer got into your medication and ingested Volatren (diclofenac) gel.
Diclofenac is a nonsteroidal anti-inflammatory drug that we don't use in dogs due to increased incidents of side effects including lethargy, tremors, convulsions, difficulty breathing, abdominal pain, gastrointestinal ulcers, diarrhea, heart rhythm abnormalities including a slow heart rate, or low blood pressure. the LD50 in dogs is a dose greater than 55mg/kg of body weight. That means 50% of dogs given that dose die.
If your pup is 40 pounds (or 18kgs), a LD50 for her is 990mgs. If she ate 1% gel depending upon the size of the tube it may not be deadly for her, but she could have some pretty significant vomiting, diarrhea and abdominal pain and cramping, possible stomach ulsers, and it may be affecting her heart rate and blood pressure, thus the lethargy.
Given the effects of this drug I would recommend a veterinary visit.
If that absolutely isn't possible today to try to protect her stomach lining today you can give either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one half of a 20mg tablet per 20 to 40 pounds of body weight every 24 hours.
These will reduce stomach acid and should help protect her stomach. I would use one of these for the next 10 -14 days or so to try and help her stomach heal
Then feed a homemade diet of 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats and juices drained off the meat and 2/3 boiled, white rice.
Ideally it is best to have her admitted to a veterinary clinic to be given oral activated charcoal to absorb any remaining drug and decrease its absorption into her body, placed on fluids, and given injectable drugs and oral drugs to reduce nausea and protect against ulcer formation.
If you choose not to have her seen today watch her closely for abdominal pain, vomiting blood, loss of appetite, passing dark tarry stools or extreme lethargy and have him seen by a veterinarian if those occur.
Please let me know if you have any further questions.