They’re a huge source of microplastics in the environment. Tl;Dr they readily break down into microplastics and small plastic filaments.

pubmed.ncbi.nlm.nih.gov/34139503/

FTA:

Cigarette butts are dangerous pieces of plastic, but are usually not handled properly and consist of more than 15,000 detachable strands of plastic fiber. Discarded cigarette butts may be carried into rivers and lakes, and finally into the ocean. The plastic fibers will continuously release microplastic fibers into the environment. About 300,000 tons of potential microplastic fibers may enter the aquatic environment from this source per annum. Additionally, toxic substances, such as nicotine, carcinogenic tar, and polycyclic aromatic hydrocarbons, have strong toxic effect, which will cause serious damage to aquatic organisms.