A food watchdog claims medicines do not always carry the correct warnings about the potential side-effects of the E-numbers they contain.
Some colourings and preservatives can cause allergic reactions or painful skin rashes in a number of people.
But a Food Commission survey found that 31 out of 41 medicines it surveyed failed to warn of the "unwanted effect" of certain additives.
EU guidelines say ingredients known to have side-effects must be listed on warning labels of medicines taken orally.
These rules apply to the E-numbers when they appear in medicines, but not when they are used in food or drink, the Food Commission said.
Ian Tokelove, spokesman for the Food Commission's magazine, which carried out the study, said: "For many people the additives appear to pose no immediate risk, but better labelling would ensure that susceptible adults and children would at least have a chance of identifying and avoiding the additives that may cause harm."
The Medicines and Healthcare products Regulatory Agency said additives are vital ingredients in many medicines, and added that labelling requirements are set out in EU and national law.
A spokesman said: "A particular requirement is that excipients of known effect are listed on the outer packaging with a related warning included within the patient information leaflet."