People who choose not to eat animal products may be unaware that common medicines could contain them, a study suggests.
Many tablets and liquid medicines use gelatin, derived from animal bones or skin.
A survey in the Postgraduate Medical Journal shows a quarter of patients are unknowingly prescribed drugs containing gelatin contrary to their beliefs.
The report authors say clearer drug labelling is needed.
A spokesman for the ABPI, which represents the pharmaceutical industry in the UK, said that European Union legislation required the disclosure of all ingredients in the leaflet accompanying the drugs.
He added: "Patients are able to check if a product contains a material which may cause them concern. If patients are unsure if an ingredient is derived from animals they can seek the advice of their pharmacist or contact the company manufacturing the product.
"There is a general trend for manufacturers to move away from the use of animal derivatives in medicines but there remain occasions where the nature of the product, or quality or safety issues, preclude the effective use of non-animal-derived ingredients."
There are many ingredients in tablets, capsules and other medicines which, while usually not part of the active treatment, help hold it together or thicken liquids.
Gelatin is commonly used, particularly in generic medicines - versions of a drug mass-produced more cheaply once the initial patent has expired.
The Manchester Royal Infirmary researchers, led by a consultant urologist, surveyed 500 patients, and found that approximately 40% in their inner-city catchment said they preferred not to consume animal products in their day-to-day lives, either because they were vegetarians, or for other cultural and religious reasons.
Of the 200 following a restricted diet in this way, 49 were found to be already taking drugs which contained gelatin, despite their wishes.
While many of the 200 said they would be prepared to take a drug containing animal products if there was no alternative, the report authors said that more effort should be made to help them, by both doctors and pharmacists.
They wrote: "In particular, we would recommend that every doctor needs to be aware that it is not just the active drug being dispensed but a whole group of other agents which may have relevance to an individual patient's compliance with treatment when oral treatments are prescribed."
They suggested that drug companies could adopt an "ingredients" list similar to that found on food packaging, or even use recognised symbols such as those promoted by the Vegetarian Society.
In addition, they said that plenty of vegetarian alternatives to gelatin were available and should be considered by manufacturers when formulating their products.
Liz O'Neill, spokesman for The Vegetarian Society, said that it frequently received calls from patients concerned about animal products in their medication.
She said that current labelling made it hard for them to make an informed choice.
"Some vegetarians will be shocked to learn about the widespread use of animal ingredients in medicines.
"This is a complex area with no overnight solutions, but the Vegetarian Society believes that everyone has a right to know what they are consuming."