Blue-green algae and the toxins they produce were detected in the lake at concentrations that could be harmful to humans and animals.
People should avoid swallowing any water or inhaling water droplets that could result from water activities like boating or water skiing.
Though the toxins are not absorbed through the skin, people with sensitive skin could get a red rash from the water.
Health officials also recommend fish harvested from the lake be thoroughly cleaned with fat, skin and organs removed before cooking.
OHA said exposure to the toxins can result in a variety of symptoms including numbing and dizziness that could lead to difficulty breathing or heart problems. Children and pets are at particular risk.
The advisory will be lifted when there is no longer a concern, according to OHA.
For health information or to report an illness, contact the Oregon Health Authority (OHA) at 971-673-0400 or Klamath County Health Department (Ramona Quinn) at 541-883-1122.