Environmental Health officers from Argyle and Bute Council issued the warning following two incidents on the Cardross shoreline.
One dog has died and another is unwell after eating a wild plant on the shoreline on Tuesday.
A council spokesman said: ""Environmental Health officers in Helensburgh, in conjunction with Amenity Services and Police Scotland, are investigating a report that one dog has died and one dog is unwell after apparently eating a wild plant on the Cardross shoreline on Tuesday 7 January 2014.
"The plant appears to be established rather than being washed ashore but this is yet to be confirmed.
"Members of the public are asked to be vigilant whilst out walking their dogs and to be aware that poisonous plants do grow wild in the UK. Do not touch or allow children or pets to come into contact with any plant growing wild you are unsure about."