Dr Kinesh Patel and Dr Kate Tatham say most medications prescribed in primary care contain animal derived products and it is unclear whether they are suitable for vegetarians.
They call for improved labelling, similar to those on food, to help inform doctors, pharmacists and patients about the content of medicines. And they stress that concerned patients should not stop taking their medication without consulting their doctor first.
Specific dietary preferences regarding animal products in food are common in the general population. Influences such as religion, culture, economic status, environmental concern, food intolerances, and personal preferences all play a part in the foods that people choose to consume.
Yet many patients and doctors are unaware that commonly prescribed drugs contain animal products - and simply reading the list of ingredients will not make it clear whether the product meets the patient's dietary preferences.
Problem ingredients include lactose (often extracted using bovine rennet), gelatine (sourced from cows, pigs and occasionally fish) and magnesium stearate (traditionally sourced from cows, pigs and sheep) although some manufacturers now use vegetarian alternatives.
Last year a campaign to vaccinate children in Scotland against influenza was halted because of concern in the Muslim community about pork gelatine within the vaccine.
Even though the absolute levels of animal products in many medications are likely to be minimal, the authors say doctors need to consider this when prescribing "to avoid non-adherence, which is a major healthcare concern."
To ascertain the scale of the problem, they identified the 100 most commonly prescribed drugs in UK primary care in January 2013. Of these, 73 contained one or more of lactose, gelatine, or magnesium stearate. But they found that information on the origins of the contents was difficult to obtain, unclear, inconsistently reported, and sometimes incorrect.
"Our data suggest that it is likely that patients are unwittingly ingesting medications containing animal products with neither prescriber nor dispenser aware," they write.
They call for improved drug labelling, mirroring those standards advised for food. However, they acknowledge it is unlikely that any labelling standard could address all dietary requirements, "and the ultimate solution would be to eliminate animal derived products where possible from medications."
They point out that lactose is already produced by some manufacturers without using rennet, magnesium stearate can be made chemically without animal ingredients and vegetarian capsules to replace gelatine are already available.
"Although vegetarian friendly ingredients may be more expensive than those produced by traditional processes, the costs would diminish as production expanded and they would limit the exposure of patients to products they find unacceptable," they conclude.