Increased Carbon Dioxide
Smoke of all kinds, including cigarette smoke, increases the amount of carbon dioxide where it is present (since smoke is itself produced via combustion). Plants use carbon dioxide to make sugars (during photosynthesis). Its increased abundance, therefore, makes it easier for the plant to obtain. Thus, smoking near a plant actually makes it easier for that plant to obtain much-needed carbon dioxide.
At the same time, however, cigarette smoke hampers the plant's ability to carry out photosynthesis itself. Over time, the particles from the cigarette smoke coat the surface of the leaf, stunting the photosynthesis process, which relies on the plant's ability to absorb sunlight from its leafy surfaces. Some plants have especially fuzzy leaves, however, and these are less affected by this slow coating.
Clogging Stomatal Pores
Aside from the smoke, the tar that cigarettes release into the air is particularly damaging to plants. This tar works to clog the stomatal pores of plants, located on the leaves. These pores are the gateway into which both carbon dioxide and oxygen are brought into the plant, the first for purposes of photosynthesis and the second for purposes of respiration. Cigarette smoke, therefore, is detrimental to the proper functioning of both these processes.