In questioning the value of medicinal marijuana, Carly Fiorina said “we don’t understand how it interacts with other drugs.” In fact, there is information about marijuana’s interactions with other medications, although major clinical trials using marijuana have been limited by its legal status.

Fiorina spoke with Fox News’ Sean Hannity, who asked her about the legal status of marijuana in Colorado (it also has been legalized in Washington). She said she believes in states’ rights but that legalizing marijuana is a “bad idea” (at the 30:20 mark):

Fiorina, June 25: I remember when I had cancer, my doctor asked me if I had an interest in medicinal marijuana. I did not, and he said good, because marijuana is a very complex chemical substance now, we don’t understand how it interacts with other drugs, we don’t understand what it does to your body.

The claim regarding drug interactions is not correct: Prescribing information for approved versions of medicinal marijuana does contain drug interaction information, and studies have turned up few problems in terms of interactions with cancer therapies, as well as other types of medication.

To be clear, there is still much to be learned about potential interactions with cannabis; its status as a schedule 1 drug makes it more difficult to study in clinical trials than other medications, meaning there is simply less information available. As a paper in the journal Pharmacotherapy in 2013 put it: “Because medical cannabis is not controlled or regularly used in mainstream medicine, the actual drug – disease and drug – drug interaction profiles remain to be elucidated.” That paper goes on, however, to list a number of interactions that are in fact known.

Some of these can be found in the prescribing information for the two approved forms of medicinal marijuana, Cesamet and Marinol. These contain synthetic versions of THC, a cannabinoid that is marijuana’s main ingredient, delivered in pill form. Fiorina was diagnosed with breast cancer in 2009, well after both of these agents had been approved by the Food and Drug Administration (which did so as far back as 1985, with subsequent revisions). However, some of the studies we note below have been published in the years since her diagnosis. We contacted Fiorina’s campaign to ask for clarification on her comments, but we did not receive a reply.

The prescribing information for Cesamet, with the active ingredient nabilone, contains warnings regarding a number of potential interactions. These include additive depressive effects when combined with any central nervous system depressant and decreased clearance of drugs from the body when taken with barbiturates. Combined with antidepressants, nabilone could cause some cardiovascular problems including tachycardia (an elevated heart rate) and high blood pressure.

The prescribing information for Marinol (active ingredient dronabinol) is similar with regard to drug interactions. It warns that the agent “should be used with caution in patients receiving concomitant therapy with sedatives, hypnotics or other psychoactive drugs because of the potential for additive or synergistic [central nervous system] effects.” It also lists antidepressants, barbiturates and other agents that may have interactions with cannabinoids in general.

Some studies also have examined what potential effects the use of medicinal marijuana agents may have specifically on patients being treated for cancer, which was the context of Fiorina’s remarks. For example, one study published in The Oncologist in 2007 showed that medicinal cannabis delivered via herbal tea “does not significantly influence” how the drugs docetaxel and irinotecan function. Docetaxel is used to treat many cancers, including certain types of breast cancer, while irinotecan is most commonly used in colorectal malignancies.

Other studies have shown that cannabinoids do in fact interact with cancer drugs — synergistically, meaning they actually add to the beneficial effects. In one 2011 study published in Molecular Cancer Therapeutics, the combination of cannabinoids and temozolomide, a drug used to treat brain tumors, produced a strong anti-tumor effect in the very difficult-to-treat brain malignancy known as glioblastoma multiforme. In another study, published in 2011 in Cell Death & Disease, the same thing was observed in pancreatic cancer when medicinal marijuana was combined with gemcitabine, which is used in many cancers.

Outside of cancer, other beneficial effects have been seen in combination with other drugs. A study published in Clinical Pharmacology and Therapeutics in 2011 showed that adding vaporized cannabis to opioids actually improved the control of chronic pain. The authors wrote that this could allow treatment of pain with lower doses of opioids, with fewer side effects.

In another field, a study showed that smoked or pill-form marijuana had no adverse effect on patients with HIV/AIDS, in spite of worries that the drug may interact with protease inhibitors used to prevent the virus from replicating in these patients. That study was published in Annals of Internal Medicine in 2003.

There is still much to be learned about medicinal marijuana, but it’s not accurate to say that “we don’t understand how it interacts with other drugs.”

Editor’s Note: SciCheck is made possible by a grant from the Stanton Foundation.

– Dave Levitan