Toledo issued the warning early Saturday. It said tests at one treatment plant returned two sample readings for microsystin above the standard for consumption.
The city warned against boiling because it will only increase the toxin's concentration. The advisory covers city residents and those in Lucas County served by the city's water supply.
The city's advisory says Lake Erie may have been affected by a bloom of harmful algae that produces the toxin. Consuming the tainted water could result in vomiting, diarrhea and other problems.
Toledo is home to about 280,000 people. The city says more tests are being run.
