One such medicine, the hormone levonorgestrel, was found in higher concentrations in the blood of fish than in women who take the contraceptive pill. Elevated levels of this hormone can lead to infertility in fish.
The study has been published in the journal Environmental Science and Technology.
In the study, the fish were exposed to treated waste water from three sewage treatment works in Stockholm, Umea and Gothenburg.
The study showed that levonorgestrel - which is found in many contraceptive pills, including the morning-after pill - can impact on the environment and constitutes a risk factor for the ability of fish to reproduce. Levonogestrel is designed to mimic the female sex hormone progesterone and is produced synthetically.