Researchers in China discovered that a common anxiety medication called diazepam is showing up in freshwater fish, and the levels are increasing. Between 2020 and 2024, they found the drug in more fish samples and at higher concentrations than before. The contamination appears to come mainly from fishing bait. While the risk to adults eating these fish seems low, children eating large amounts could potentially face health concerns. This study highlights the need for better control of what goes into fishing areas to keep our food supply safe.

The Quick Take

  • What they studied: Whether a medication used to treat anxiety (diazepam) is building up in freshwater fish and whether eating these fish could harm human health
  • Who participated: Fish samples collected from freshwater areas in Zhejiang Province, China over a 5-year period (2020-2024), with special attention to recreational fishing areas
  • Key finding: The amount of diazepam found in fish increased significantly over the study period—from being detected in less than 1% of samples to nearly 5% of samples, with concentrations rising 75% on average. Some fish contained very high levels (up to 76 micrograms per kilogram)
  • What it means for you: If you eat freshwater fish from these areas, your exposure to this medication appears to be low and unlikely to cause harm for most adults. However, children who eat large amounts of contaminated fish could potentially be at risk, suggesting parents should be mindful of fish consumption amounts for young children

The Research Details

Scientists collected fish samples from freshwater areas in Zhejiang Province, China over five years and tested them for the presence of diazepam and related compounds. They measured how much of the drug was present in different fish species and tracked whether levels were increasing or decreasing over time. They also tested fishing bait to understand where the contamination was coming from. Additionally, they used computer modeling to understand how the drug interacts with fish biology at a molecular level.

The researchers paid special attention to recreational fishing areas, where they suspected contamination might be worst. They identified which fish species accumulated the most medication and calculated how much of the drug a person might consume by eating these fish. They then compared these exposure levels to safety standards to determine if there was a health risk.

Understanding how medications end up in fish and whether they accumulate to dangerous levels is important for food safety. This research approach helps identify the source of contamination (in this case, fishing bait) so that targeted solutions can be developed. By studying which fish species are most affected, authorities can provide better guidance about which fish are safer to eat.

This study tracked contamination over a 5-year period, which is long enough to identify real trends rather than random fluctuations. The researchers used multiple methods to understand the problem, including chemical testing, source identification, and health risk calculations. However, the study focused on one region of China, so results may not apply everywhere. The specific number of fish samples tested was not clearly stated in the available information, which makes it harder to assess how representative the findings are.

What the Results Show

The most striking finding was a dramatic increase in diazepam contamination over the five-year study period. In 2020, the drug was detected in only 0.33% of fish samples, but by 2024, it was found in 4.67% of samples—a more than 14-fold increase. The average amount of the drug in contaminated fish also grew significantly, rising 75% from about 2.31 micrograms per kilogram to 4.04 micrograms per kilogram.

Recreational fishing areas showed the worst contamination, suggesting these areas are particularly vulnerable. Crucian carp (a common food fish) accumulated the most medication compared to other fish species, making it the highest-risk fish to eat. The researchers found that fishing bait was the primary source of contamination, with some bait samples containing extremely high levels of diazepam (up to 110,000 micrograms per kilogram).

When the researchers looked at metabolites (breakdown products of the medication), they found that one compound called nordiazepam remained relatively stable at low levels, while other breakdown products were rarely detected. This suggests that fish bodies process the medication in specific ways.

Computer modeling revealed that diazepam and its breakdown products bind to specific receptors in fish brains differently depending on the fish species, which helps explain why some fish accumulate more of the drug than others. Crucian carp showed the highest bioaccumulation factor (a measure of how much the fish concentrates the drug from its environment), at 1238.60, meaning these fish concentrate the drug over 1,200 times more than their surroundings.

This is one of the first comprehensive studies tracking diazepam contamination in freshwater fish over an extended period. Previous research has documented pharmaceutical contamination in aquatic environments, but this study provides detailed information about how the problem is worsening and which fish species are most affected. The finding that fishing bait is the primary source is particularly important because it suggests a controllable point of intervention.

The study focused only on one region of China (Zhejiang Province), so the findings may not apply to other parts of China or the world. The exact number of fish samples tested was not clearly reported, making it difficult to assess how confident we should be in the results. The study did not measure actual human health effects, only estimated potential exposure based on fish consumption patterns. Additionally, the research assumed typical eating patterns and may not account for people who eat much larger or smaller amounts of fish.

The Bottom Line

For most adults, eating freshwater fish from these areas appears to pose low risk based on current contamination levels (moderate confidence). Parents should be aware that children eating large amounts of freshwater fish from recreational fishing areas could potentially face health concerns and may want to limit children’s consumption of these fish or choose alternative protein sources (moderate confidence). Authorities should implement stronger regulations on fishing bait quality and contamination control in recreational fishing areas (high confidence).

Parents of young children should pay attention to this research, especially if they live in or frequently visit Zhejiang Province or similar areas with recreational fishing. People who catch and eat their own fish from these waters should be aware of potential contamination. Public health officials and environmental regulators should prioritize controlling the source of contamination. Adults with typical fish consumption patterns face minimal risk based on current data.

If contamination sources are not controlled, the problem will likely continue to worsen based on the 5-year trend observed. If regulations are implemented to control fishing bait quality, improvements in fish safety could potentially be seen within 1-2 years. Any health effects from current exposure levels would likely develop over months to years of regular consumption, not immediately.

Want to Apply This Research?

  • If you live in an affected area and eat freshwater fish regularly, track your weekly fish consumption by type (crucian carp vs. other species) and source (recreational fishing areas vs. markets). Note any unusual symptoms and correlate with fish intake patterns.
  • Users can reduce potential exposure by: (1) limiting crucian carp consumption to 1-2 times per week instead of daily, (2) choosing fish from commercial sources rather than recreational fishing areas when possible, (3) varying fish species consumed, and (4) ensuring children eat smaller portions of freshwater fish.
  • Track fish consumption patterns monthly and note any changes in local fish contamination news. If you catch your own fish, consider having bait tested or switching to bait from regulated commercial sources. Monitor local environmental health advisories for updates on fish safety in your area.

This research describes contamination of freshwater fish in Zhejiang Province, China, and should not be interpreted as medical advice. If you have concerns about medication exposure or health effects, consult with a healthcare provider. This study estimates potential health risks but does not prove that eating contaminated fish will cause illness. Individual risk depends on how much fish you eat, which species you consume, and your age and health status. Always follow local health department guidelines regarding fish consumption. If you experience symptoms you believe are related to food consumption, seek medical attention immediately.