The assisted suicide organisation, Dignitas, has lost an appeal to be permitted to store natrium-pentobarbital for use by patients wishing to die.
Switzerland's highest court ruled on Wednesday that natrium-pentobarbital falls under the law on narcotics.
It said that permission could be granted to national and international organisations to use the substance in emergencies to preserve life, but this could not be extended to Dignitas.
It explained that Dignitas is a private organisation and wants to use the narcotic to take life rather than save it.
Meanwhile a poll to be published in the French-language magazine Hebdo on Thursday shows that three quarters of people in Switzerland are "fairly or greatly" in favour of assisted suicide, and that 56 per cent would consider using it if they were suffering from a serious incurable disease.
However, opinions were almost equally divided as to whether foreigners should be allowed to travel to Switzerland in order to die.
swissinfo with agencies