CANTON, Ill. (AP) _ A 79-year-old great-grandmother says she's bringing President Reagan's war on drugs home to Canton, beginning with the nursing home where she lives.

Ruth Howard said she got so excited watching the president and Mrs. Reagan's televised call for a national crusade against illegal drug use that she hardly slept Sunday night.

Monday morning, she rallied fellow residents and staff at the Nursing Center of Canton in a campaign to plaster the home with posters reading, ''Just Say No,'' the slogan of Nancy Reagan's national campaign against drug abuse.

Mrs. Howard said she hung one of the posters on her wheelchair, and planned to hit the streets with the message.

''We're getting ready to put posters all over the city,'' she said in a telephone interview Monday. ''I'm going to ask any business with a mirror to put them up, like barbershops. I don't suppose it'd do any good in taverns, but it can't hurt anything.''

Mrs. Howard said she hopes to get the message into the schools in this central Illinois city of 14,000.

''I want 'Just Say No' written on all the blackboards in all the schools all the time,'' she said. ''If I can just stop one child from taking drugs in school, I will have accomplished something worthwhile.''

''I think it's super,'' said administrator Kathy Demler. ''She's as active as she can be, especially when she's excited about something.''

The home is more a starting point than a target, Mrs. Howard said, adding that her main concern is stopping children from experimenting with drugs.

''I have one great-grandchild who is 13 years old, and I worry terribly about it,'' she said.