Research suggests that many reported side effects that occur with CBD use are most often the result of drug-to-drug interactions between CBD and other medications an individual may be taking. That's why it's so important to speak with your doctor before starting any supplement, and this is particularly true with CBD.
Here's how these interactions occur: CBD is broken down by the body via the same pathway as many prescription drugs. If multiple compounds are competing in this pathway (e.g., CBD and a prescription drug), then something called "altered concentration" can occur. This means that too little or too much of the drug is left in the body. When too little remains, a drug may no longer work as intended. When too much remains, side effects may increase. Penn State College of Medicine has a really handy list of medications that may be impacted by altered concentration due to a combination with cannabinoids. Unfortunately, this list does not make any distinctions between CBD or THC, but it provides some guidance. Bring this list to your doctor before starting any CBD product.
Penn State also found potentially serious interactions between prescription CBD and THC products and the following products:
Warfarin and other blood thinners
Amiodarone (heart medication)
Levothyroxine (thyroid medication)
Seizure medications (clobazam, lamotrigine, valproate)