“It’s hard to walk into a dispensary right now and really trust that the products are safe,” said Kamani Jefferson, president of the Massachusetts Recreational Consumer Council, which represents marijuana consumers. “They can point to the label and say it’s been tested, but until the state steps in and sets some standards, they can’t really say it with a straight face.”

Setting rules on the quality of marijuana is inherently challenging. While the plant has been the subject of some medical research, decades of prohibition mean there are no established standards for how much and what kinds of contamination might be considered dangerous. Some states have looked to standards for produce, but those are of limited value, since marijuana is more frequently smoked or vaporized than eaten.

In Massachusetts, the Department of Public Health set relatively tight limits for medical marijuana on microbial contamination from mold, yeast, and bacteria, amid concerns that patients with compromised immune systems could be susceptible to infection from tainted cannabis products.

The debate between the labs in the state stems from differences in how they test marijuana for microbes.

Two of the labs — Proverde Laboratories in Milford and MCR Labs in Framingham — use a traditional “plating” technique, in which samples of cannabis are placed in a petri dish containing a gel-like medium that encourages the growth of microbes. After an incubation period, technicians count how many visible spots, or “colonies,” of mold or other contaminants have grown in the gel, and use a standard formula to calculate whether that means the marijuana is unacceptably contaminated according to health department limits.

Two newer labs — CDX Analytics in Salem and EVIO Labs in Southbridge — employ a faster, DNA-based technique called qPCR. Using expensive mass spectrometers and chromatography machines, the companies say, they can precisely measure microbial contamination by detecting how many times microbes’ genetic codes divide.