Cannabidiol (CBD) interactions with prescription drugs are theoretically possible, but as of yet, they are largely under-researched and scientifically unproven. That said, CBD interaction with medicine still poses—as it should—a serious consideration for potential users. This is mostly because the cannabinoid gets metabolized by the same enzymes in the liver as many prescription drugs, which means that the latter's efficacy and potency can be affected. While uncommon, such CBD interactions can be dangerous, even fatal.
Below, we start with a list of red-alert prescription medicine—definitely consult with your doctor first before taking CBD concomitantly with these medicines.
Also read on for some information on the specific enzymes involved in CBD metabolism, as well as more on CBD's safety profile and which side effects you may experience using CBD.
Drugs Known for THC and CBD Interaction
The Columbia Department of Health made available a document (Medical Cannabis Adverse Effects & Drug Interactions) discussing and listing the most prominent drugs that CBD and tetrahydrocannabinol (THC) can potentially interact with.
THC, the other best researched cannabinoid found in Cannabis sativa, gets mentioned here because many excellent CBD cannabis products contain both. They work well together and also with other compounds such as terpenes, etc.
The following was taken from the Columbia DoH's document. 
In theory, CBD may increase serum (blood) concentrations of:
- macrolides (bacteriostatic antibiotics)
- calcium channel blockers, as well as beta blockers (prescribed to lower blood pressure and treat heart rhythm irregularities and angina)
- benzodiazepines (anxiolytic)
- cyclosporine (immunosuppressant)
- sildenafil (and other PDE5 inhibitors, which treat impotence or erectile dysfunction)
- antihistamines (to treat allergies)
- antiretrovirals (HIV-suppressor)
- some statins (atorvastatin and simvastatin, but not pravastatin or rosuvastatin. Statins all lower cholesterol)
- antidepressants including SSRIs and tricyclics
- antipsychotics (including haloperidol)
- opioids and opiates (including morphine, codeine, and oxycodone, all highly addictive analgesics)
In theory, THC can decrease blood concentrations of:
- clozapine (sedative to treat schizophrenia)
- duloxetine (anxiolytic antidepressant)
- naproxen (anti-inflammatory)
- cyclobenzaprine (muscle relaxant)
- olanzapine (treating mood and mental disorders, antidepressant)
- haloperidol and chlorpromazine (antipsychotics)
Again, it needs to be stressed that this THC and CBD interaction is mostly theoretical, as it is based on these cannabinoids' metabolic activity in the liver.
CBD and THC in the Liver
The above list is based on research that has shown THC to be a CYP1A2 enzyme inducer, while CBD appears to be a potent inhibitor of CYP3A4 and CYP2D6 enzymes. Both inhibit CYP2C9 enzyme activity.  
Clinically Demonstrated Interactions
But not all is theory.
Clinical research has shown CBD to increase serum levels of warfarin, a blood thinner—also topiramate and rufinamide, both anti-epileptics. It also increased desmethylclobazam—a metabolite of clobazam, another antiepileptic—in the blood. Yet paradoxically, it decreased clobazam serum levels.  
Some research has furthermore shown THC to affect blood glucose levels, as well as theophylline, a bronchodilator. This doesn't seem to happen via the enzymatic pathway. 
Who Should Therefore Avoid Taking CBD or CBD with THC without a Doctor's Recommendation?
Obviously, if you're taking any of the mentioned medications, it is imperative to visit your doctor before starting on CBD.
All cannabis use is contraindicated in the following conditions, except when supervised by a medical doctor. Most of these effects are dose-related.
- Anyone suffering from high blood pressure, as cannabis can drop blood pressure (BP) between 10 to 20 and up to 40 points. This could cause dangerously low BP, which could result in fainting spells, disorientation, severe lightheadedness, and worse.
- Any person diagnosed with ischemic heart disease, as they are already battling with low BP.
- Those battling cardiac arrhythmia or rapid heart rate (tachycardia). The latter can be a side effect caused by other drugs, and using CBD with THC can exacerbate the symptom. Cannabis use concomitant with anticholinergic antidepressants is completely contraindicated for this reason.
- Anyone suffering from epilepsy.
- Anyone using a blood thinner.
- Any person diagnosed with a severe mental disorder, such as schizophrenia, should probably avoid using any product with THC. CBD might be safe, but again—only if administration is monitored by a psychiatrist.
- Very high CBD doses in those with liver disease or impairment.
- By pregnant and breastfeeding mothers.
- Children and teenagers should not be given THC-high products unless strictly under medical supervision. Also, teens should abstain from smoking weed, as its use has been associated with brain development and mental health issues as well as developmental learning problems.
RELATED: CBD Mood Stabilizers—Is It a Thing?
Is CBD Safe?
All of this may paint a scary picture and could put anyone off. Yet while caution is warranted, there's no reason not to explore CBD as a supplement to the mentioned medications. It may even be a good thing, as your doctor could titrate you down to a lower dose, especially of chronic prescription medicine. Lower doses could reduce the side effects these meds are most often associated with, which may even inspire your compliance with taking them. Again—always consult with your prescribing doctor first, as it will be crucial in some cases to monitor your serum drug levels.
Furthermore, CBD health benefits are numerous, and the cannabinoid has been shown to be safe for use in humans. It is not chemically addictive, and it doesn't alter the mind like THC in weed.
Side effects are usually transient and self-correcting. They also tend to occur more when the CBD oil doses are very high and the user is cannabis-naive. These symptoms can include:
- Sleepiness and drowsiness
- Nausea and vomiting (especially in the case of high-dose THC. However, high-THC cannabis products are unlawful unless prescribed by a doctor, and then you would usually need to enroll in your state's medical cannabis program. Consult with your state department of health for particulars, as these differ nationwide.)
- Dry mouth
- Changes in mood and appetite
- Decreased fertilization capacity 
CBD health effects are numerous, as research increasingly demonstrates. Don't let CBD drug interactions hold you back since the effects are usually dose-related and can be managed with the help of a medical doctor.