The government has warned that drugs sold over the counter or bought over the internet are creating a serious problem of addiction and are often the first choice for drug users.
Speaking in a Lords debate on the world drug problem, Baroness Masham of Ilton, a member of the all-party Parliamentary group on drug misuse, said she could not see that "making all drugs legal would solve the problem of drug abuse, as we have seen the problem of addiction to over-the-counter and prescribed drugs which are legal".
Speaking about a recent inquiry into prescription drug addiction she said: "Drugs containing narcotics and/or psychotropic substances have effects similar to illicit drugs when they are taken in inappropriate quantities and without medical supervision."
According to the inquiry, 30,000 people may depend on drugs containing codeine, with middle-aged women most at risk, showing that "addiction to over-the-counter painkillers is becoming a serious problem".
The inquiry found that some people take more than 70 pills a day, dramatically increasing the risk of liver dysfunction, stomach disorders, gallstones and depression.
Baroness Masham argued that being able to bulk-buy supplies of drugs makes it difficult for doctors to know about the problem, meaning that addicts are rarely given adequate support.