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Paraquat Still Testing Positive in U.S. Marijuana Seizures

From a 2004 Freedom of Information Act (FOIA) request to the National Institute on Drug Abuse (NIDA), ProCon.org obtained documents indicating that the U.S. government, through sub-contractors, is still testing marijuana for Paraquat, a toxic pesticide that was banned in the United States because of its harmful health effects, and finding it in some seized marijuana samples.

Annual reports (11/9/99 - 11/8/03) from Mahmoud A. ElSohly, Ph.D., Director of the NIDA Marijuana Project at the National Center for Natural Products Research, School of Pharmacy, University of Mississippi, revealed the following:

YearNo. of Seized Samples AnalyzedControl* Samples AnalyzedTotal Samples AnalyzedNo. of Samples Positive for Paraquat% of Seized Samples with Paraquat
2000-2001Not stated**Not stated**1,060373.49%**
*The report did not describe what "control" meant, although it appears that marijuana grown by the University of Mississippi is used as a "control" that does not contain Paraquat.
** The report for this year included only the total samples analyzed, and did not distinguish between types of samples.

Added as an attachment to the Annual Report of 2002-2003 was the following analysis [Analysis in PDF]:

"Analysis of Paraquat in Confiscated Marijuana Samples

Report #32

November 9, 2002 through November 8, 2003

During this report period a total of 400 samples were analyzed constituting 400 seizures and 80 controls. Of these 400 samples, 137 were from Southwest Regional Laboratory, of which 2 were positive (1.46%). The remaining 263 samples were from South Central Regional Laboratory with 11 positives (4.18%)

Overall, the qualitative analysis showed the rate to be 13 positive samples among the 400 seizures or 3.25% positive.

All controls were correctly identified and there were no false positives from the analysis of the negative controls.

Attached [PDF version of attachment] is a detailed list of the positive samples analyzed in this report [inset below]."

PM Number*Seizure DateSeizure PlaceAmount Seized (Kg)
1.499337/10/02Laredo, TX162,660.000
2.503348/6/02Ranchita, CA845.400
3.504279/26/02Laredo, TX94,170.000
4.5042810/1/02Laredo, TX8,700.000
5.504329/26/02Falfurrias, TX44,400.000
6.506709/19/02El Paso, TX528,448.200
7.509791/10/03Del Rio, TX253,670.000
8.511511/21/03Zapata, TX866,180.000
9.511531/24/03Laredo, TX192,090.000
10.5116310/22/02Eagle Pass, TX65,499.000
11.514073/1/03Honolulu, HI672.800
12.514502/22/03Laredo, TX280,550.000
13.514563/8/03Laredo, TX1,848,420.000

* The annual reports noted that "All confiscated samples received by the Project for analysis are labeled Potency Monitoring (PM) samples.... This includes domestic samples as well as samples supplied by the [Drug Enforcement Administration] DEA Regional Laboratories. These samples were analyzed in duplicate for verification..."

[Note: A similar breakdown was not provided for the other annual reports we received.]

General Health Information on Paraquat

A 5/8/03 Fact Sheet from the U.S. Center for Disease Control and Prevention, Department of Health and Human Services, states:

"If it is inhaled, paraquat could cause poisoning leading to lung damage. In the past, some marijuana in the United States has been found to contain paraquat." [Fact Sheet in PDF format]

Medline Plus, a service of the U.S. National Institutes of Health, states on its website in a 1/12/03 entry:

"Paraquat lung is a lung disease caused by the weed killer Paraquat (dipyridylium).

Causes, incidence, and risk factors

Paraquat is a weed killer once promoted by the United States for use in Mexico to destroy marijuana plants. Research found that this herbicide was dangerous both to workers who applied it to the plants and to people who smoked the marijuana harvested from them.

Paraquat is now banned in the United States, although it is still legal for some uses in Mexico. As most marijuana smoked in the U.S. is now domestically grown, the risk of paraquat lung to American marijuana smokers is minimal.

Paraquat may cause lung damage by inhalation, it may cause esophageal erosions or ulcers from ingestion and absorption through the gut, or it can cause skin breakdown through contact with intact skin.

Severe ingestion of Paraquat can rapidly cause death from respiratory failure due to a form of proliferative alveolitis, or inflammation of the air spaces in the lungs. This impairs oxygen transport to the blood and may lead to acute respiratory distress syndrome (ARDS).

Administering oxygen to improve the blood oxygen level may actually increase the damage to the lung in such cases of ARDS. Chronic exposure to Paraquat, may cause pulmonary fibrosis, or a stiffening of the lung tissue.

Paraquat may also damage the kidneys, liver, mouth, and esophagus. Death may occur from perforation of the esophagus, or from acute inflammation of the mediastinum, the area that encases major blood vessels and airways in the middle of the chest.


  • shortness of breath
  • difficulty breathing
  • breathing requires increasing effort
  • sore throat
  • vomiting or GI distress
Signs and tests
  • history of exposure to Paraquat
  • history of marijuana smoking in the 1970's
  • arterial blood gasses (abnormal results)
  • X-ray of the chest
  • electrolytes (see chem-20)
  • BUN
  • creatinine
  • pulmonary function tests
  • urine toxicology screen

There is no specific treatment for Paraquat poisoning. The goal is to relieve symptoms and address any complications that arise.

Expectations (prognosis)

The outcome depends on the severity of exposure. The affected person may develop mild breathing-related symptoms with full recovery, or they may have permanent changes in the lungs, or if the person received a large dose at once (as with ingestion), death may occur.


  • ARDS
  • Pulmonary fibrosis
  • Esophageal perforation
  • Mediastinitis

Calling your health care provider

If you believe you have been exposed to Paraquat, you should seek medical care immediately."