The practice of combining cannabis with other psychoactive substances is known as polysubstance use. This behavior introduces complex and often unpredictable interactions that go beyond the effects of either substance consumed alone. When two or more substances are present, they can engage in chemical synergy, meaning the combined effect is greater than the sum of their individual effects. Understanding these interactions is important because cannabis compounds, particularly delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD), affect the body’s central nervous system and metabolic pathways, which can significantly alter the action of other drugs.
Combining Cannabis with Alcohol and Other Depressants
The combination of cannabis and central nervous system (CNS) depressants, such as alcohol, produces a synergistic increase in impairment. Both substances slow down brain activity, affecting coordination, judgment, and reaction time, and when used together, these effects are often multiplicative rather than simply additive. This interaction is frequently referred to as “crossfading,” and it can lead to a state of profound intoxication.
Alcohol can increase the absorption of THC into the bloodstream, resulting in higher concentrations of the cannabis compound than if cannabis were consumed alone. This elevated exposure to THC can rapidly trigger a severe reaction known as “greening out,” characterized by intense nausea, dizziness, sweating, and vomiting. The heightened impairment from combining cannabis with alcohol also increases the risk of accidents and poor decision-making.
Mixing cannabis with other prescribed CNS depressants, like benzodiazepines or opioids, intensifies sedation and motor impairment. This deeper level of CNS depression raises concerns about respiratory function and the ability to maintain consciousness.
Combining Cannabis with Stimulants
Mixing cannabis with stimulants, such as cocaine, amphetamines, or high doses of caffeine, creates a different set of risks, primarily centered on cardiovascular strain and psychological distress. Stimulants increase heart rate (tachycardia) and blood pressure, and THC can also independently cause these effects. When combined, the drugs produce additive effects on the heart, significantly increasing the rate-pressure product, which is a measure of cardiac workload.
This increased cardiovascular demand can lead to palpitations, chest pain, and a heightened risk of acute cardiac events, particularly in individuals with pre-existing heart conditions. Psychologically, the combination can elevate anxiety and paranoia. Stimulants increase central nervous system activity, and THC can overstimulate the amygdala, the brain region that regulates fear-related emotions. Stimulants may also mask the sedating effects of cannabis, leading to a false sense of sobriety and potential overconsumption of the stimulant.
Co-use with Nicotine
The co-use of cannabis and nicotine, often by mixing tobacco into joints or using blunts, introduces unique dependency and respiratory concerns. The presence of nicotine significantly increases the addictive potential of the combination. Inhaling tobacco smoke or nicotine vapor along with cannabis exposes the lungs to a greater concentration of harmful byproducts, which is associated with chronic bronchitic symptoms and other respiratory issues.
Interactions with Prescription Medications
A distinct safety concern arises from the interaction between cannabis compounds and the liver’s drug-metabolizing enzymes. The body uses the Cytochrome P450 (CYP450) enzyme system to process and eliminate most prescription drugs. Both major cannabinoids, THC and CBD, can inhibit the function of several key CYP450 enzymes, including CYP3A4 and CYP2C9.
When cannabis inhibits these enzymes, the body processes co-administered medications more slowly, leading to a buildup of the drug in the bloodstream. This can result in dangerously elevated concentrations, increasing the risk of adverse side effects and toxicity. For example, the metabolism of the blood thinner warfarin, which is processed by CYP2C9, can be inhibited by cannabis, potentially causing an increased risk of bleeding.
Conversely, an interaction can also make a medication less effective if the pharmaceutical is meant to be activated by an enzyme that cannabis inhibits. Drug classes commonly affected by this metabolic interference include certain anti-epileptic medications, antidepressants, and immunosuppressants used for organ transplants. Because of the unpredictable nature of these metabolic interactions, patients taking any long-term prescription medication should consult a healthcare provider before combining it with cannabis.