Introduction: Ketoprofen is one of the nonsteroidal anti-inflammatory drugs most used in the population, and has an epitope similar to 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid, so it can be a cause of false positives to marijuana. Objective: Assess whether ketoprofen at therapeutic doses is a cause of false positive in the detection of Δ9-tetrahydrocannabinol in urine by immunoassay. Methods: A cross-sectional, experimental uncontrolled two-phase sampling, convenience sampling and prospective recruitment study included 40 volunteers (26 female and 14 male) over 18 years old. The Advanced Quality One Step Cannabinoid (THC) test was used. 50 mg of ketoprofen were administered, after 7 days 100 mg of the same drug was administered, in both phases at a single dose, orally, fasting and with 250 mL of water. Urine samples were collected at 0, 1, 2, 4, 6, 9 and 12 hours. Results: At doses of 50 mg of ketoprofen, false positives were observed in urine samples from the first hour in women, and from 2 hours (female n = 16; 61.5 %; male n = 9; 64.3 %) to 6 hours in both sexes; at doses of 100 mg of the same drug, false positives were observed, from the first (female n = 21; 80.8 %; male n = 9; 64.3 %) to the sixth hour, in both sexes. Urine samples were false positive at 6 hours (100%) for both sexes and at the two doses studied. Conclusions: Ketoprofen induces cross-reactivity of false positives to the dose-independent Advanced Quality One Step Cannabinoid (THC) test, and in case of requiring a test for Δ9-tetrahydrocannabinol, it should be known if the subject has previously consumed ketoprofen, to perform the test in a time not less than 9 hours of having consumed the drug. At the same time, all positive screening tests for Δ9-tetrahydrocannabinol should be confirmed by more accurate techniques.