CBD (Cannabidiol) is being used for dogs suffering from osteoarthritis pain, epilepsy (Seizure Disorders), and anxiety.
CBD cannabidiol is one of over a 100 cannabinoids or phytocannabinoids. (naturally occurring cannabinoids from plants) “CBD is extracted from cannabis plants and processed as an isolate or as a full-spectrum oil combined with other related cannabinoids. CBD oils made with full-spectrum extracts are thought to have superior therapeutic effects versus cannabidiol-only oil. 
CBD can be refereed to as: (amounts vary greatly)
Hemp oil Does NOT contain CBD
There are two common strains of cannabis in use today — hemp and marijuana. Hemp is a cannabis plant that contains less than 0.3% of THC; marijuana has more than 0.3%. CBD products for pets are typically sourced from hemp.”  “CBD is thought to have many therapeutic properties, but unlike cannabis’s other main compound THC (tetrahydrocannabinol), it doesn't get users high.”  THC as found in marijuana can be toxic for dogs and cats so its important that any CBD product contain less than 0.3% THC. For comparison a 0.56 oz (16 g) edible cookie made from cannabis (marijuana) contains about 10 mg of THC which makes one cookie toxic for pets.  If your dog has ingested marijuana, call your veterinarian immediately or the Animal Poison Control Center (APCC) at (888) 426-4435.
There have been published studies on CBD use in dogs for osteoarthritis and epilepsy with promising results but for anxiety there are only anecdotal reports to date.   Dr. Rob Silver, a veterinarian, herbalist and pioneer in the field reports “one of the most common applications for CBD is for anxiety and behavior issues in dogs. It seems to work very well at low dosages for hyper dogs.” 
“If you live in a state that has legalized CBD, there’s nothing to stop you from giving it to your pet on your own. But experts say it’s a good idea to talk with your vet first.”  CBD does interact with some drugs and can have side affects so it is recommended to consult your veterinarian before giving it to your pet. Casara Andre, D.V.M., founder of Veterinary Cannabis Education & Consulting, “says, CBD can interact with medications, including those used by vets. Also, there’s less oversight and testing of products marketed for pets, and there have been reports of animals harmed by contaminated or spoiled CBD products, she says. There have also been reports of animals that seemed to get high from products, possibly because the formulations had more THC than was claimed. (CBD-only products are supposed to contain less than 0.3 percent THC.)” 
Buyer beware CBD products have been referred to as the wild west. “It can be hard to pick the best CBD oil because product labels often don't list what they contain and, even then they do, many have been shown to be wrong.” CBD products are unregulated and you may not be getting any CBD in the product you purchase and some have major health concerns which could be dangerous. (See Fox 11 video) “cannabis plants readily absorb heavy metals, pesticides, and other potentially harmful chemicals that may be in the soil or water”  CBD products have also been found to be contaminated with a dangerous synthetic cannabinoid. 
“Our tests of CBD products revealed that two products contained less CBD than listed and one contained significantly more. Actual doses ranged from just 1.3 mg to 22.3 mg, and we found that you can't rely on listed amounts of "cannabinoids" to tell you how much CBD is in a product.” https://www.consumerlab.com/reviews/cbd-oil-hemp-review/cbd-oil/
How to Choose
It’s recommended you start your search at consumerlab.com before purchasing a brand or choose one of the trusted brands below. Another trusted resource for researching any pet supplement is the National Animal Supplement Council (NASC)
Osteoarthritis: Cornell researchers used a CBD product made by ElleVet Sciences.
Epilepsy: Dr. McGrath at Colorado State used a CBD product manufactured by Applied Basic Science Corporation
Look for the Product’s Certificate of Analysis
“For any CBD product for you or your pet, your best bet is to find a company that has commissioned independent third-party testing and can provide a Certificate of Analysis, or COA.”  “The lab results should show how much CBD (and THC) the product contains, as well as how the product did in tests checking for contaminants such as heavy metals and fungicides, Cital says. If you can’t find a COA on the company’s website or the company refuses to share it, that's a red flag.” 
“What to look for? “look for products that list the amount of CBD or cannabidiol per serving (and don’t confuse that with the amount per entire bottle). If a product lists only “cannabinoids” it may contain some CBD but you won’t know how much. Products may still have a significant amounts of CBD if they list “hemp extract” as an ingredient, but don’t expect much CBD if “hemp oil” is the only ingredient.” https://www.consumerlab.com/reviews/cbd-oil-hemp-review/cbd-oil/
There can be a wide range in the total amount of CBD in each product. You cannot take the information presented at face value without determining the total CBD or cannabidiol. Sometimes the number represents the total amount of cannabinoids (full spectrum oil) and not the CBD or cannabidiol content. To keep it clear I view the cannabinoids (full spectrum oil) or “noids” like vitamins and minerals. When refering to CBD / cannabidiol or “diol’s”, I view this like a particular vitamin or mineral. “noids” are general and “diol’s” are the specific CBD content which is our primary focus.
A 1 ML dropper can have 1mg or 50mg or more of CBD. The only way to compare is to know the total amount of CBD per ML (one dropper full) or the total amount of CBD in the bottle. This may take some research at the brands website if the information is not on the bottle or box. The blue bottle below displays “75 MG” on the bottle and for this product it represents there is 75 mg total of CBD in the 2oz bottle. This works out to 1.25 mg of CBD for each full dropper of 1 ML. The box to the right shows “Hemp Oil 1700”. This product has 500 mg of CBD per 1oz bottle working out to 17 mg for a full dropper of 1 ML.
How long will a bottle last? That depends on the dose which can vary greatly. Dosages reported range from 0.1 mg per kg of body weight, up to 2 mg per kg (and higher) of body weight as in the osteoarthritis clinical study recently completed at Cornell. One kg is equal to 2.2 lbs.
A 50lb dog receiving 2 mg per kg (2.2 lbs) of body weight twice a day.  (90 mg per day)
1.25mg ml 0.83 days $49.99 x 1.2
8mg ml 2.6 days $24.95
15mg ml 4.1 days $95
17mg ml 5.6 days $89.99
30mg ml 10 days $95
50mg ml 16.6 days $109.95 - $119.95
A 50lb dog receiving 1 mg per 10 pounds of body weight twice a day.  (10 mg per day)
1.25mg ml 7.5 days $49.99
8mg ml 24 days $24.95
15mg ml 37.5 days $95
17mg ml 51 days $89.99
30mg ml 90 days $95
50mg ml 150 days $109.95 - $119.95
Work with a veterinarian and choose carefully. And keep your animals away from all forms of marijuana.
“it’s important to understand the difference between medicinal marijuana for humans versus pets. It’s primarily about THC levels — humans can tolerate a significantly higher level of THC than is safe for pets.”
“Marijuana for humans contains much more THC than pets can safely consume; CBD oil for pets is made from hemp plants and contains very low levels of THC, which removes the risk of toxicity” “The Pet Poison Helpline reported a stunning 448 percent increase in calls for marijuana poisoning over the past six years, with the majority involving pets who ingested marijuana-laced food products.”
A “clinical study recently completed at Cornell suggests that 2 mg/kg of cannabidiol (CBD) oil twice daily can help increase comfort and activity in dogs with osteoarthritis.” “The study was headed by Joe Wakshlag, DVM, PhD, DACVN, DACVSMR, associate professor and section chief of nutrition at Cornell.“ (ElleVet was the CBD oil used in this recent study at Cornell)
Preliminary data from CBD clinical trials ‘promising’ (for dogs with epilepsy)
“When it comes to pain management, there's a significant amount of evidence that supports the use of CBD. And since cannabinoids work differently in the body than narcotics and also non-steroidal anti-inflammatory drugs, they can be used together without the risk of drug interactions.”
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A recommended resource for selecting pet supplements is the National Animal Supplement Council (NASC)
“NASC Primary Suppliers are responsible suppliers of animal health and nutritional supplements for dogs, cats, and horses. These companies are committed to quality, vigilance, and continuous improvement to promote the wellbeing of companion animals. Primary Suppliers must sign and abide by the NASC Code of Conduct when they join. Primary Suppliers must complete an independent facility audit and comply with the NASC’s stringent quality requirements to earn permission to display the NASC Quality Seal on their products.” https://nasc.cc/members/