Runners at north Wales' largest running event were warned they would be disqualified if they dropped litter during the race.
Organisers of Sunday's Conwy Half Marathon said the measures had been introduced as plastic was becoming an "increasing problem".
The Run Wales website said runners would also be "taken off the results if seen discarding their rubbish outside of a water stop or not with a marshal".
About 3,000 runners took part.
Organisers have not yet said if anybody had been disqualified following the race, which started in front of the town's castle at 10:00 GMT.
Run Wales is following a growing trend among race organisers to reduce the environmental impact of events.
In April 2019, the London Marathon trialled several measures to reduce litter after clearing 47,000 plastic bottles from the streets in 2018.
These included using compostable cups rather than plastic bottles at some stations along the route, and using plastic bottles made wholly or partly from recycled plastic.
This year's Cardiff Half Marathon used 100% recyclable plastic bottles, recycled paper for all print advertising and medals made from recycled zinc.