Combining Cannabis With Amphetamines Did Not Make Impairment Worse Than Cannabis Alone
When young adults used cannabis and dextroamphetamine together, the combination produced impairment similar to cannabis alone, without the added stimulant making performance worse or better.
Quick Facts
What This Study Found
In mice, THC enhanced locomotor activity and amplified the stimulant effects of methamphetamine. But in human participants, the picture was different.
When young adult males smoked cannabis (25 micrograms/kg THC) and took oral dextroamphetamine (10 mg/70 kg), their psychomotor performance and subjective effects were not significantly different from cannabis alone. The amphetamine did not rescue cannabis-induced impairment in standing steadiness or attentive motor performance.
Dextroamphetamine alone, even at doses that raised blood pressure, produced no significant performance impairment. But participants themselves believed their driving ability was impaired under the combination, a judgment the researchers noted was supported by their lab measurements.
Key Numbers
THC doses: 3, 6, or 9 micrograms/kg via smoking. Dextroamphetamine doses: 5, 10, or 15 mg/70 kg orally. Combination dose: 25 micrograms/kg THC + 10 mg/70 kg d-AMP. Systolic blood pressure was elevated at the 15 mg d-AMP dose.
How They Did This
The study combined animal experiments (mice, locomotor activity) with human testing. Young adult males received graded THC doses via marijuana smoking (3, 6, or 9 micrograms/kg) and dextroamphetamine (5, 10, or 15 mg/70 kg). Performance was measured via standing steadiness, attentive motor tasks, delayed auditory feedback, and subjective evaluations.
Why This Research Matters
Polysubstance use is common, and understanding how cannabis interacts with stimulants matters for assessing real-world impairment. This study suggested amphetamines do not counteract cannabis-induced motor impairment, which challenges the assumption that stimulants might "cancel out" depressant effects.
The Bigger Picture
This was early evidence that drug interactions involving cannabis are not always additive or synergistic. The finding that a stimulant failed to reverse cannabis impairment has implications for understanding polydrug use patterns, particularly among people who might assume one substance can offset the effects of another.
What This Study Doesn't Tell Us
The doses used were described as "modest." The sample consisted only of young adult males. Animal and human results diverged, highlighting species differences. The study did not assess cognitive effects beyond psychomotor performance.
Questions This Raises
- ?Would higher doses of either substance produce synergistic impairment?
- ?Do other stimulants (caffeine, cocaine) interact differently with cannabis?
- ?How do these findings translate to real-world driving scenarios?
Trust & Context
- Key Stat:
- Amphetamine did not rescue cannabis-induced impairment in motor tasks
- Evidence Grade:
- A controlled study with multiple dosing conditions, but modest doses and male-only participants limit generalizability.
- Study Age:
- Conducted in 1976. Cannabis potency and amphetamine formulations have changed substantially.
- Original Title:
- The combined effect of marihuana and dextroamphetamine.
- Published In:
- Annals of the New York Academy of Sciences, 281, 162-70 (1976)
- Authors:
- Forney, R, Martz, R(2), Lemberger, L(2), Rodda, B
- Database ID:
- RTHC-00012
Evidence Hierarchy
Participants are randomly assigned to treatment or placebo groups to test cause and effect.
What do these levels mean? →Frequently Asked Questions
Did amphetamines make cannabis impairment worse?
No. The combination produced impairment roughly equivalent to cannabis alone, neither worse nor better.
Did participants feel impaired?
Yes. Most subjects believed their driving performance would be impaired under the combination, and laboratory measurements supported that perception.
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Cite This Study
https://rethinkthc.com/research/RTHC-00012APA
Forney, R; Martz, R; Lemberger, L; Rodda, B. (1976). The combined effect of marihuana and dextroamphetamine.. Annals of the New York Academy of Sciences, 281, 162-70.
MLA
Forney, R, et al. "The combined effect of marihuana and dextroamphetamine.." Annals of the New York Academy of Sciences, 1976.
RethinkTHC
RethinkTHC Research Database. "The combined effect of marihuana and dextroamphetamine." RTHC-00012. Retrieved from https://rethinkthc.com/research/forney-1976-the-combined-effect-of
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.