Patients are being urged to exercise caution and find out what's in their medicines as new drug labelling rules roll out.

People suffering food allergies will get more information about what's in their medicines with drug companies forced to redesign their labels to disclose potentially deadly ingredients.

The Therapeutic Goods Administration says unlike foods, most medicines aren't obliged to reveal all the ingredients on the label, even if they contain substances that could cause an allergic reaction in some people.

Last year, it gave pharmaceutical companies four years to redesign their labels.

The TGA on Thursday instructed doctors to ensure patients with less common allergies know that some allergens will still not be covered under the new rules.

It's warned of potential confusion for patients during the transition phase, given it will be difficult to tell which labels follow the old rules or the new rules before the September 2020 deadline.

"Therefore, during this transition, it is important to keep asking your doctor, pharmacist or other health professional about food allergen content in medications," the TGA said.

Prescription medicines will also be required to declare allergens for the first time but are allowed to provide the allergen information on the consumer medicine information leaflet accompanying the pack.

Under existing rules, companies were already forced to declare allergens like peanuts and gluten.

The new rules require a longer list of allergens to be declared, including soya, milk, tree nuts, eggs, fish and crustacea.