Anecdotal evidence of cannabis use by professional programmers abounds. Recent studies have found that some professionals regularly use cannabis while programming, even for work-related tasks. However, accounts of the impacts of cannabis on programming vary widely and are often contradictory. For example, some programmers claim that it impairs their ability to generate correct solutions, while others claim it enhances creativity and focus. There remains a need for an empirical understanding of the true impacts of cannabis on programming. This paper presents the first controlled observational study of cannabis’s effects on programming ability. Based on a within-subjects design with over 70 participants, we find that, at ecologically valid dosages, cannabis significantly impairs programming performance. \textbf{Programs implemented while high contain more bugs and take longer to write ($p<0.05$)} — a small to medium effect ($0.22 \leq d \leq 0.44$). We also did not find any evidence that high programmers generate more divergent solutions. However, programmers can accurately assess differences in their programming performance ($r=0.59$), even when under the influence of cannabis. We hope that this research will facilitate evidence-based policies and help developers make informed decisions regarding cannabis use while programming.