A. Is CBD Dangerous?
1. What the FDA Says about Its Safety
2. What Is Warfarin?
3. An Interaction Between Warfarin and Cannabidiol, a Case Report
4. Probable Interaction Between Warfarin and Inhaled and Oral Administration of Cannabis
5. But Can CBD Thin My Blood?
B. The Other Dangers of Taking CBD
C. In Conclusion
Cannabidiol (CBD), a constituent of Cannabis sativa L., is suddenly all the rage in most states across the U.S.
The recent removal of hemp cannabis from the U.S Food and Drug Administration (FDA)'s list of controlled substances and various state-managed medical marijuana programs are probably the most prominent reasons for this recent explosion of the market.
But is CBD really safe and effective—especially for people who take blood-thinning medicine? Is it an anticoagulant by itself?
Some consumers are still wondering about this.
The country's major medicine-regulating body, the FDA, leans toward a verdict of "It can be dangerous because we don't know enough about how it works yet."
This is the warning posted on their site: 
CBD has the potential to harm you, and harm can happen even before you become aware of it.
This is quite evocative and alarmist language, with very little backing scientific evidence that CBD does indeed "have the potential to harm you"—and cause liver damage, for instance.
Also, scroll down on their page for a bit and you will read that the FDA worries about the long-term effects of CBD on the developing brain—i.e., children's brains. Yet only last year, the organization approved the first epilepsy drug for...
...use in children to treat seizures caused by Lennox-Gastaut syndrome or Dravet syndrome. It can be given to children ages 2 years and older. 
Sooooo...kids with epilepsy don't have to worry about this safety concern of the FDA’s? We are wondering about this apparent double standard.
But theoretically, because of the way cannabis is metabolized, CBD can interfere with the efficacy and safety of some drugs. In fact, there is some evidence that one type of meds—blood thinners (warfarin or Coumadin/Jantoven)—can potentially be affected by CBD when taken together.
Healthline explains the purpose of this medicine:
Warfarin is used to treat and prevent blood clots that might result in heart attack, stroke, or death. It’s also used for blood clots in atrial fibrillation, heart valve replacement, venous thrombosis, and pulmonary embolism. 
Warfarin also has a so-called black box warning from the FDA, according to Healthline.
This is the most serious warning from the Food and Drug Administration (FDA). A black box warning alerts doctors and patients to potentially dangerous effects.
They go on to explain that warfarin thins the blood and limits its ability to clot.
It can cause serious bleeding, which can lead to death. You must have regular blood tests and visits with your doctor to monitor your condition. Don’t start or stop any other drug or herbal product unless your doctor tells you to. Tell your doctor if you have any signs or symptoms of bleeding.
So it's clear that blood thinners and their side effects are not to be messed with. Too much of it in the bloodstream could be fatal, so it makes sense to ask whether CBD can interfere with its efficacy.
A search online for research in this regard brings up very few human studies. We could source only two. Let's take a look at them to see if there's real reason for concern.
A clinical case report or case study is a specific type of academic publication, sometimes submitted by medical practitioners themselves. In this, they share facts about unusual patient cases or cases that haven’t been described before. 
The most recent report of a CBD-warfarin interaction was a case study published in 2017. 
The patient was a 44-year-old Caucasian male with Marfan Syndrome (MFS). MFS is a genetic disorder of the connective tissue. Patients present with exceptionally long limbs, fingers, and toes. They also tend to experience complications that involve the heart and aorta, with an increased risk of mitral valve prolapse and aortic aneurysm. 
The patient in question had to have mechanical mitral valve replacement done and was on warfarin therapy. He also suffered from post-stroke epilepsy.
He was enrolled in the University of Alabama, Birmingham open-label program for compassionate use of CBD for the management of treatment-resistant epilepsy.
The seizures started when the patient was 27 years old, after cardiac surgery, when he experienced a stroke. He did fine on one anti-seizure medicine for years, but in 2011, the seizures returned and couldn't be managed even with multiple medicines. Epilepsy surgery was considered, but the man was determined to be a poor surgical candidate for various reasons.
At the time of study enrolment, the patient was taking two antiepileptics, both twice daily. He was also taking warfarin, with a goal International Normalized Ratio (INR) of 2–3. Before he started taking CBD, his INR has been stable for approximately six months.
He was started on a dose of 5mg of CBD per kilogram of body weight twice a day. Per study protocol, the dose was doubled every two weeks. Toward the end of the program, he was taking approximately 1800mg of CBD twice a day.
This is what happened:
With up-titration of CBD oil, a non-linear increase in the INR was noted (Table 1, Fig. 1). Warfarin dosage adjustments were made by primary care physician in (an) effort to maintain an INR within his therapeutic range. At the most recent study visit, his warfarin dose had been reduced by approximately 30%. The patient was followed clinically without bleeding complications.
Here's what we think is important to notice about this case study:
The authors of this study concluded the following:
This finding suggests an interaction between warfarin and cannabidiol, underscoring the importance of monitoring appropriate laboratory work in patients receiving concomitant cannabis products and other pharmaceuticals, particularly those metabolized by the cytochrome P450 enzyme system. In patients taking warfarin, INR monitoring is suggested during initiation and up-titration of cannabinoids.
It's imperative to consult with your doctor first if you want to take CBD while taking any blood-thinning medication.
The positive spin is that, based on this study, taking CBD concomitantly with warfarin could be a good thing. CBD seems to increase the levels of the medicine that’s in your blood, so you might need less of it over time.
Under no circumstances should you try this on your own, however, because your doctor will need to adjust the warfarin based on regular blood tests, which you probably already know.
Again, this case study demonstrated elevated INR after only one month of ingesting edible cannabis and smoking joints. These only dropped after the patient discontinued cannabis use.
The authors were even more conservative in their conclusion regarding the possible drug-drug interaction of cannabis.
There are no previous reports of interactions between edible cannabis and warfarin, with very few case reports describing the interaction with other forms of cannabis. Close monitoring of INR in patients with concomitant cannabis is recommended for proper warfarin management. 
The truth is that we just don't know yet. Scientists think it might be the case, but research into this specifically is scant at best.
And the data is confusing right now.
For instance, one study looked at the anticoagulant effects of three cannabinoids (THC, CBD, and CBN) in obese rats. They concluded that only THC and CBN displayed anticoagulant activity. 
It should be clear, therefore, that a lot more well-designed clinical study is needed in this regard.
(Again, don't be tempted to experiment by yourself with CBD for anticoagulation purposes.)
Any other dangers? Well, so far, no deaths due to CBD consumption have been reported anywhere in the world.
But there was one bad incident.
In 2018 in Utah, 50 people fell ill after taking a CBD product sold in the stores. Some had to be hospitalized, and they experienced symptoms that included altered mental status, vomiting, seizures, and shaking.
Let this go to show that knowing what's in the product you're buying is very important, though. Because this poisoned product was a synthetic CBD, with no trace of the real thing inside. 
(Make sure you buy products where the contents have been third-party lab-tested and the report is easily accessible.)
The truth is simply that more data indicates CBD's excellent safety and side effect profile than the opposite.
A recent updated survey of the literature on the safety and side effects of CBD reinforced this.
The comprehensive review of 132 original studies by Bergamaschi et al. describes the safety profile of CBD, mentioning several properties: catalepsy is not induced and physiological parameters are not altered (heart rate, blood pressure, and body temperature). Moreover, psychological and psychomotor functions are not adversely affected. The same holds true for gastrointestinal transit, food intake, and absence of toxicity for non-transformed cells. Chronic use and high doses of up to 1500 mg per day have been repeatedly shown to be well tolerated by humans.
Nonetheless, some side effects have been reported for CBD, but mainly in vitro or in animal studies. They include alterations of cell viability, reduced fertilization capacity, and inhibition of hepatic drug metabolism and drug transporters (e.g., p-glycoprotein).1 Consequently, more human studies have to be conducted to see if these effects also occur in humans. In these studies, a large enough number of subjects (has) to be enrolled to analyze long-term safety aspects and CBD possible interactions with other substances. 
In layman's terms, this is what's important to take away from the above:
Serious side effects have mainly been reported in laboratory or animal studies, meaning that this has not been seen in humans so far. They include:
Some say that "you cannot argue with dead mice," but this assertion is flawed. Human physiology differs from animal physiology in important ways. For instance, CBD binds to different receptors in the animal central nervous system than in the human one. (In fact, this may account for studies’ different outcomes in terms of CBD’s anticoagulant properties. It will have to be tested, though.)
There's a lot more evidence that CBD is safe for use than the opposite. With low-dose CBD, there is no indication of serious drug interaction to date. Only in the very high-dose range (which, in the real world, is not what most people ingest every day) and in isolated cases has cannabidiol shown possible interaction with blood thinners.
The evidence that it is a blood thinner on its own is also far from conclusive.
To stay on the safe side, though—if you're on any medicine, don't take CBD or cannabis products without consulting your doctor first.
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