Drug Interaction Report

MONITOR: Coadministration of cannabidiol with other agents known to induce hepatotoxicity may potentiate the risk of liver injury. Cannabidiol causes dose-related elevations of liver transaminases, both alanine aminotransferase (ALT) and aspartate aminotransferase (AST). In controlled studies, the incidence of ALT elevations above 3 times the upper limit of normal (ULN) was 13% with cannabidiol versus 1% with placebo, and 17% in patients taking cannabidiol 20 mg/kg/day compared to 1% taking 10 mg/kg/day. Less than 1% of cannabidiol-treated patients had ALT or AST levels greater than 20 times the ULN. Some cases required hospitalization. In clinical trials, serum transaminase elevations typically occurred within the first two months of treatment initiation, but up to 18 months were reported in some cases, particularly in patients taking concomitant valproate. Resolution occurred with discontinuation or dosage reduction of cannabidiol and/or concomitant valproate in about two-thirds of the cases. In about one-third of the cases, transaminase elevations resolved during continued cannabidiol treatment, without dose reduction. The majority of ALT elevations occurred in patients taking concomitant valproate. Concomitant use of clobazam also increased the incidence of transaminase elevations, but to a lesser extent. In cannabidiol-treated patients, the incidence of ALT elevations greater than 3 times the ULN was 30% in patients taking both concomitant valproate and clobazam, 21% in patients taking concomitant valproate (without clobazam), 4% in patients taking concomitant clobazam (without valproate), and 3% in patients taking neither drug. Insufficient data are available to assess the risk of concomitant administration of other hepatotoxic drugs. Finally, patients with baseline transaminase levels above the ULN also had higher rates of transaminase elevations during cannabidiol treatment. In patients taking 20 mg/kg/day in controlled trials, the frequency of treatment-emergent ALT elevations greater than 3 times the ULN was 30% when ALT was above the ULN at baseline, compared to 12% when ALT was within the normal range at baseline. No patient taking cannabidiol 10 mg/kg/day experienced ALT elevations greater than 3 times the ULN when ALT was above the ULN at baseline, compared with 2% of patients in whom ALT was within the normal range at baseline.

MANAGEMENT: Caution is advised if cannabidiol is used in patients who are currently receiving or have recently received treatment with other hepatotoxic agents, and vice versa. Serum transaminases and total bilirubin levels should be obtained prior to initiating cannabidiol, and patients with elevated baseline transaminase levels above 3 times the ULN accompanied by elevations in bilirubin above 2 times the ULN should be evaluated. Repeat levels should be obtained at 1 month, 3 months, and 6 months after initiation of cannabidiol treatment, and periodically thereafter or as clinically indicated (e.g., within 1 month following changes in cannabidiol dosage or addition of/changes in medications that are known to impact the liver). Consider more frequent monitoring of serum transaminases and bilirubin in patients who are taking valproate or who have elevated liver enzymes at baseline. Patients who develop clinical signs or symptoms suggestive of hepatic dysfunction (e.g., unexplained nausea, vomiting, right upper quadrant abdominal pain, fatigue, anorexia, jaundice, dark urine) should have serum transaminases and total bilirubin measured promptly, and cannabidiol treatment interrupted or discontinued as appropriate. Cannabidiol should be discontinued in patients with elevations of transaminase levels greater than 3 times the ULN and bilirubin levels greater than 2 times the ULN. Patients with sustained transaminase elevations of greater than 5 times the ULN should also have treatment discontinued. Patients with prolonged elevations of serum transaminases should be evaluated for other possible causes. Also consider dosage adjustment or discontinuation of any coadministered medication that is known to affect the liver.

GENERALLY AVOID: Chronic, excessive consumption of alcohol may increase the risk of acetaminophen-induced hepatotoxicity, which has included rare cases of fatal hepatitis and frank hepatic failure requiring liver transplantation. The proposed mechanism is induction of hepatic microsomal enzymes during chronic alcohol use, which may result in accelerated metabolism of acetaminophen and increased production of potentially hepatotoxic metabolites.

MANAGEMENT: In general, chronic alcoholics should avoid regular or excessive use of acetaminophen. Alternative analgesic/antipyretic therapy may be appropriate in patients who consume three or more alcoholic drinks per day. However, if acetaminophen is used, these patients should be cautioned not to exceed the recommended dosage (maximum 4 g/day in adults and children 12 years of age or older).

ADJUST DOSING INTERVAL: Food may affect the plasma concentrations of cannabidiol. In healthy volunteers, administration of cannabidiol with a high-fat/high-calorie meal increased cannabidiol peak plasma concentration (Cmax) by 5-fold and systemic exposure (AUC) by 4-fold and reduced the total variability compared with administration in the fasted state.

GENERALLY AVOID: Grapefruit juice may increase the plasma concentrations of cannabidiol. The proposed mechanism is inhibition of CYP450 3A4-mediated metabolism of cannabidiol by certain compounds present in grapefruit. The interaction has not been studied, but the product labeling for cannabidiol recommends consideration of a dosage reduction when used with strong or moderate inhibitors of CYP450 3A4. In general, the effect of grapefruit juice is concentration-, dose- and preparation-dependent, and can vary widely among brands. Certain preparations of grapefruit juice (e.g., high dose, double strength) have sometimes demonstrated potent inhibition of CYP450 3A4, while other preparations (e.g., low dose, single strength) have typically demonstrated moderate inhibition.

MANAGEMENT: Cannabidiol should be taken about the same time each day consistently either with or without food. Patients should limit the consumption of grapefruit and grapefruit juice. If they are coadministered, cannabidiol levels should be monitored and the dosage adjusted as necessary.

MONITOR: Smoking cessation may lead to elevated plasma concentrations and enhanced pharmacologic effects of drugs that are substrates of CYP450 1A2 (and possibly CYP450 1A1) and/or certain drugs with a narrow therapeutic index (e.g., flecainide, pentazocine). One proposed mechanism is related to the loss of CYP450 1A2 and 1A1 induction by polycyclic aromatic hydrocarbons in tobacco smoke; when smoking cessation agents are initiated and smoking stops, the metabolism of certain drugs may decrease leading to increased plasma concentrations. The mechanism by which smoking cessation affects narrow therapeutic index drugs that are not known substrates of CYP450 1A2 or 1A1 is unknown. The clinical significance of this interaction is unknown as clinical data are lacking.

MANAGEMENT: Until more information is available, caution is advisable if smoking cessation agents are used concomitantly with drugs that are substrates of CYP450 1A2 or 1A1 and/or those with a narrow therapeutic range. Patients receiving smoking cessation agents may require periodic dose adjustments and closer clinical and laboratory monitoring of medications that are substrates of CYP450 1A2 or 1A1.

Theoretically, grapefruit juice may increase the plasma concentrations of loratadine as it does other drugs that are substrates of the CYP450 3A4 enzymatic pathway. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. The clinical significance of this potential interaction is unknown. Reported interactions with potent CYP450 3A4 inhibitors like clarithromycin, erythromycin and ketoconazole have produced substantial increases in the area under the plasma concentration-time curve (AUC) of loratadine and its active metabolite, descarboethoxyloratadine, without associated changes in the overall safety profile of the drug.