BRAZIL · HEALTH
Key Facts
- —What is happening Researchers at the University of São Paulo medical school (FMUSP) found fentanyl in the saliva of 21 of 65 synthetic cannabinoid users in São Paulo, none of whom said they took opioids.
- —Why it matters Fentanyl drives the US overdose crisis, and Brazil has long had little non-medical opioid use, so a hidden exposure in its biggest city is a possible early warning.
- —The numbers Fentanyl was found in about 32% of the 65 volunteers, recruited between August 2024 and August 2025; 90.5% of those positive had nausea or vomiting, against 43.2% of the rest.
- —Who is who FMUSP is the University of São Paulo’s medical school, and first author Ligia Goes Endo is a doctoral student there; K2 and K9 are street names for synthetic cannabinoids.
- —What to watch Whether further testing of the drugs themselves shows where the fentanyl came from, since the study did not analyse what the volunteers actually took.
- —Power Index Brazil ranks 1st of 14 in The Rio Times Latin America Power Index, with 75.0 points (October 2026 edition).
- —What it means for you Visitors to Brazil should know that street drugs there may hold opioids; the study found no sign of risk for ordinary travellers, but an overdose needs emergency care at once.
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A saliva study of 65 synthetic drug users in Brazil’s largest city found fentanyl in about one in three. The authors say it does not prove a city-wide trend.
Researchers at the University of São Paulo found fentanyl in São Paulo in the saliva of 32% of 65 drug users. None of the positive cases said they took opioids on purpose, NC News reported on Thursday, 22 October.
Fentanyl is a synthetic opioid used in hospitals that also drives overdose deaths in the United States. Brazil has historically had far less non-medical opioid use than North America or Europe, so the finding is being watched closely.
What the Study Found
The study appeared in the journal Drug and Alcohol Review, NC News, Correio and Voz da Bahia reported. The first author is Ligia Goes Endo, a doctoral student at FMUSP.
Fentanyl was detected in 21 of the 65 volunteers, about 32%, NC News reported. All 65 said they had used synthetic cannabinoids, sold as K2 or K9, in the four days before their interviews.
Synthetic cannabinoids are lab-made chemicals that act on the same brain receptors as cannabis, but their makeup varies widely. NC News notes their effects are not necessarily the same as marijuana’s.
CNN Brasil reported that 45 of the 65 samples, 69%, held at least one substance. It adds that 83% of the volunteers were men, mostly aged 23 to 29, and 42% were homeless or in shelters.
How Researchers Tested for Fentanyl
The 65 volunteers were recruited from August 2024 to August 2025 at three care services in São Paulo city, NC News reported. One was a treatment centre and two were community psychosocial care services.
Correio reported that the oral fluid was collected with a dedicated device and analysed by the FMUSP toxicology laboratory. None of the sources gives the laboratory method or the detection threshold.
Volunteers also answered a questionnaire on drug use and symptoms, NC News said. The saliva samples were then tested for drugs.
Symptoms and Who Was Affected
Those who tested positive more often had severe symptoms, NC News reported. Nausea or vomiting affected 90.5% of the fentanyl-positive group against 43.2% of the others.
Muscle pain was reported by 61.9% of the positive group against 11.4% of the rest. Homelessness was more common too, at 52.4% against 13.6%.
The authors found no single factor that independently predicted fentanyl in their statistical tests, NC News reported. Correio adds that unintended exposure was more common among homeless people and those with a long history of synthetic drug use.
What the Study Does Not Show
The researchers did not test the drugs the volunteers used, NC News reported. So the study cannot confirm that street drugs were laced with fentanyl.
The group is small and specific, so the study does not show that three in ten users in the city are exposed. It also does not show a fentanyl crisis in Brazil, NC News said.
Symptoms also cannot be blamed on fentanyl alone, since associations within the sample do not prove cause. Other possible explanations include undeclared opioid use and exposure by other routes.
Police Laboratory Data on Fentanyl
The Scientific Police of São Paulo state, the forensic arm of the state police, analyses drug seizures. Its 2025 report on 2024 says fentanyl and its analogues rose from 0.4% of samples in 2022 and 2023 to 2.4%.
Fentanyl alone was identified 73 times in 2024, or 2.2% of cases, the report says. It also says fentanyl and a class of potent opioids called nitazenes often appear combined with synthetic cannabinoids.
Synthetic cannabinoids were the main substance in 1,932 of 3,381 new psychoactive substance cases in 2024, or 57.1%. That sample came from 29,888 drug seizures analysed that year.
What It Means for US Readers
Americans live, study and travel in São Paulo, Brazil’s largest city and financial centre. The study does not suggest a risk for ordinary visitors, but it shows opioid exposure can occur without a person knowing.
The authors advise doctors to consider opioid exposure even when a patient says they used none, NC News reported. For suspected severe intoxication, NC News advises calling the emergency service Samu on 192.
For US readers following the opioid crisis, the finding adds a Brazilian data point to a regional picture. Related reading: LatAm’s fentanyl crisis opinion and xylazine and Mexico’s opioid crisis.
City context is in the report on São Paulo crime trends. More is on the Brazil news hub.
What Is Not Known
The journal article itself could not be opened, so the figures here come from three Brazilian outlets that reported on it. No DOI or full author list was published in those reports.
The source of the fentanyl is unknown, and the study did not analyse the drugs themselves. Laboratory method, detection threshold and the amounts found were not reported.
Some details come from one outlet only. The 69% rate, the demographics and the September publication date are CNN Brasil’s, and the symptom percentages are NC News’s.
What Comes Next
The authors call for continuous monitoring of the drug market and of biological samples. Correio quotes Endo as calling it key to harm reduction and to emergency protocols, including naloxone, a drug that reverses opioid overdoses.
No official response from the São Paulo state or city health authorities was found. That does not mean a trend has been confirmed, only that the question is now open.
Frequently Asked Questions
What did the São Paulo study find?
Researchers found fentanyl in the saliva of 21 of 65 synthetic cannabinoid users, about 32%. None of those who tested positive reported using opioids on purpose.
Does this mean 32% of São Paulo drug users have fentanyl?
No, because the group of 65 was small and specific. The authors say the evidence does not show a city-wide or national trend.
Where did the fentanyl come from?
That is unknown. The study did not test the drugs themselves, and lists lacing, undeclared opioid use and other exposure routes as possibilities.
What are K2 and K9?
They are street names for synthetic cannabinoids, lab-made chemicals sprayed on plant material or sold in other forms. Their makeup varies widely and their effects are not the same as cannabis.
What should I do if someone shows signs of an opioid overdose in Brazil?
NC News advises calling the emergency service Samu on 192 if breathing is slow, the person is unconscious or cannot be woken. Do not wait for symptoms to pass.
Sources: NC News, 22 October; Correio, 22 October; Voz da Bahia, 22 October; CNN Brasil, 22 October; Scientific Police of São Paulo, New Psychoactive Substances in São Paulo, Report 2025 (all accessed 23 October 2026).
This article was produced by The Rio Times’ automated newsroom system. How we use AI · Report an error · Editorial responsibility: Matthias Camenzind, Editor-in-Chief
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