It could be poisonous if they are made from polyvinyl chloride, or PVC.
Due to their insolubility in water and relative chemical inertness, pure plastics generally have low toxicity in their finished state, and will pass through the digestive system with no ill effect (other than mechanical damage or obstruction).
However, plastics often contain a variety of toxic additives. For example, plasticizers like adipates and phthalates are often added to brittle plastics like polyvinyl chloride (PVC) to make them pliable enough for use in food packaging, children's toys and teethers, tubing, shower curtains and other items. Traces of these chemicals can leach out of the plastic when it comes into contact with food. Out of these concerns, the European Union has banned the use of DEHP (di-2-ethylhexyl phthalate), the most widely used plasticizer in PVC. Some compounds leaching from polystyrene food containers have been found to interfere with hormone functions and are suspected human carcinogens.
Moreover, while the finished plastic may be non-toxic, the monomers used in its manufacture may be toxic; and small amounts of those chemical may remain trapped in the product. The World Health Organization's International Agency for Research on Cancer (IARC) has recognized the chemical used to make PVC, vinyl chloride, as a known human carcinogen. Some polymers may also decompose into the monomers or other toxic substances when heated.
The primary building block of polycarbonates, bisphenol A (BPA), is an estrogen-like hormone disrupter that may leach into food.. Research in Environmental Health Perspectives finds that BPA leached from the lining of tin cans, dental sealants and polycarbonate bottles can increase body weight of lab animals' offspring. A more recent animal study suggests that even low-level exposure to BPA results in insulin resistance, which can lead to inflammation and heart disease.
Bis(2-ethylhexyl) adipate, present in plastic wrap based on PVC, is also of concern, as are the volatile organic compounds present in new car smell. Toxic chemicals allegedly released by the reuse of water bottles have been the subject of urban legend. 
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