Combining Cannabis With Amphetamines Did Not Make Impairment Worse Than Cannabis Alone | Research

The takeaway

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.

Read this if you want to understand whether stimulants offset cannabis-related impairment.

Amphetamine did not rescue cannabis-induced impairment in motor tasks

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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.

What this study cannot 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.

How to read the evidence

A controlled study with multiple dosing conditions, but modest doses and male-only participants limit generalizability.

When this study was published

Conducted in 1976. Cannabis potency and amphetamine formulations have changed substantially.

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.

Questions still open

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?

Common 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.

Read the original research

The combined effect of marihuana and dextroamphetamine.

Annals of the New York Academy of Sciences, 281, 162-70

View on PubMed ↗

Citation

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.