Mixing Marijuana and Amphetamines: Heart Effects Added Up but Cognitive Impairment Came from the Weed | Research

The takeaway

Combining marijuana and dextroamphetamine produced additive increases in heart rate and blood pressure, but psychomotor impairment was driven entirely by marijuana with no additional effect from the stimulant.

Read this if you use cannabis alongside stimulants and want to understand how the combination affects your heart and cognitive performance.

Additive cardiac load
heart rate and blood pressure increased from both drugs combined, but cognition was impaired by marijuana alone

What the researchers found

In two double-blind, randomized studies, researchers tested the combination of marijuana and dextroamphetamine. Heart rate and blood pressure increases were additive when both drugs were taken together. EKG changes were nonspecific and associated with marijuana.

For cognitive and motor performance, impairment tracked with marijuana only. Adding amphetamine to marijuana did not worsen or improve psychomotor performance compared to marijuana alone. Subjective effects measured by a modified symptom questionnaire also showed only additive patterns, with no synergistic interaction.

Why it matters

This is one of the earliest controlled drug interaction studies involving cannabis. The finding that cardiovascular effects were additive but cognitive effects were not is practically important for understanding polydrug use. It suggested that mixing cannabis with stimulants increases cardiac strain without counteracting the mental impairment from cannabis.

The numbers in context

- Amphetamine dose: 10 mg/70 kg

- THC dose: 50 mcg/kg (Study 1), 25 mcg/kg (Study 2)

- Heart rate and blood pressure: additive effects from combination

- Psychomotor impairment: attributable to marijuana only

- Subjective effects: additive, no synergistic interaction

How the study worked

Two separate double-blind, randomized, complete block design studies. Study 1: placebo or 10 mg/70 kg dextroamphetamine orally, followed 1.5 hours later by a marijuana cigarette delivering 50 mcg/kg THC. Cardiovascular measures tracked. Study 2: same amphetamine dose with marijuana delivering 25 mcg/kg THC. Psychomotor performance evaluated.

What this study cannot tell us

Small sample size (not specified in abstract). Only one dose combination was tested in each study. Short-term, single-session design. Conducted in the 1970s with lower-potency cannabis than is available today. The amphetamine dose was relatively low.

How to read the evidence

Randomized, double-blind design but small sample size and single-session exposure. Preliminary evidence for drug interaction effects.

When this study was published

Published in 1976. One of the first controlled cannabis-stimulant interaction studies. The core finding remains relevant for understanding polydrug cardiovascular risk.

The bigger picture

People who use cannabis with stimulants (amphetamines, cocaine, ADHD medications) often assume the stimulant will offset the cognitive impairment from cannabis. This study suggested otherwise: the heart bears the combined load of both drugs while the brain stays impaired by the cannabis regardless of the stimulant.

Questions still open

Would higher stimulant doses overcome cannabis-induced psychomotor impairment?
Does the additive cardiovascular effect pose clinical risks for people with heart conditions who use both substances?

Read the original research

Effects of marihuana-dextroamphetamine combination.

Clinical pharmacology and therapeutics, 20(3), 350-8

View on PubMed ↗

Citation

Evans, M A; Martz, R; Rodda, B E; Lemberger, L; Forney, R B. (1976). Effects of marihuana-dextroamphetamine combination.. Clinical pharmacology and therapeutics, 20(3), 350-8.