State health authorities Friday more than doubled the amount of medical marijuana that patients can buy, a move made after regulators determined laboratories can ensure the safety of the drug.
The state Department of Public Health had imposed a stricter limit since the first medical marijuana dispensary opened in June.
Under the rules put in place Friday, patients will be allowed to buy up to 10 ounces of marijuana every two months.
“We are proud to be moving forward with science-based protocols, based on stakeholder feedback, that will allow us to continue assuring patients are receiving safe, high-quality marijuana for medical use,” said Scott Zoback, the department spokesman.
Regulators had been concerned that laboratories were not able to safely screen for levels of cadmium and other heavy metals, leaving patients vulnerable to a toxic accumulation of contaminants.
The health department granted waivers to the four marijuana dispensaries that have opened in the state, permitting them to sell the drug with higher-than-allowed contaminant levels, but in limited quantities — a maximum of 4.23 ounces over two months.
Regulators based that strict limit on worst-case scenarios, assuming patients might consume the marijuana in a short amount of time.
The restrictions prompted an outcry from patients, dispensary owners, and laboratory chemists who said the rules were too stringent and impeded access to medical marijuana.
Now, with data about medical marijuana use across the country, the state has based contaminant limits on the amount patients are expected to regularly use. Regulators will require labeling to indicate the appropriate dosage. The concept is similar to what applies to an over-the-counter painkiller such as Tylenol, which is considered safe in recommended doses but potentially harmful in greater quantities.
“By adopting this approach, we are ensuring that the standards . . . have been developed through a rigorous process with a high level of public assurance that they have been developed using a broadly representative body of science,” the department said in a letter sent Friday to dispensary owners.
The changes come as two more dispensaries are poised to open.
Patriot Care in Lowell is scheduled to open Tuesday.
New England Treatment Access in Brookline sent a notice Friday to patients that the dispensary would start filling orders by appointment only. The company faced stiff neighborhood opposition to a dispensary opening in busy Brookline Village and only won town approval by agreeing to limit parking spaces and restrict hours of operation for its first few weeks as town officials monitored the business.Kay Lazar can be reached at Kay.Lazar@globe.com. Follow her on Twitter @GlobeKayLazar.