A campaign has been launched to get members of the public to report any side-effects they experience after taking medicines.
Not all side-effects are spotted during clinical trials
The Medicines and Healthcare Regulatory Authority (MHRA) wants pharmacists to promote and make better use of the "Yellow Card" scheme.
The scheme, which includes online reporting, was set up in 1964 in the wake of the Thalidomide tragedy.
It has since helped detect dozens of unexpected side-effects.
While drugs are heavily tested prior to release, some "adverse effects" may not be spotted, or the medicine may interact with other drugs or even foods in an unexpected way.
Reports on the Yellow Card scheme helped scientists find out that cranberry juice interacted with warfarin, one of the most commonly prescribed blood thinning drugs.
In 2001, other Yellow Card reports revealed a connection between smoking cessation drug Zyban and seizures.
The MHRA also wants community pharmacists to file more reports on drug reactions to them.
Most of the 20,000 reports every year come directly from doctors, but only a few hundred from community pharmacists.
Dr June Raine, from the MHRA, said: "We are keen to let people know that whilst their medicines have important benefits, they may also have unwanted side-effects.
"If you suspect that you have had a side-effect to your medicine, please tell us about it via the Yellow Card scheme."
She said that members of the public could either pick up a reporting form from their pharmacist, file a report online, or speak to the pharmacist to help guide them through the process.
A spokesman for the MHRA stressed that they were interested to hear about any side-effect, including side-effects already mentioned by the doctor or covered by the drug packaging.
Shelley Flanagan suffered side-effects after using antibiotics to treat pneumonia.
She said: "I would encourage anyone who believes they have had a side-effect to a medicine to fill out a Yellow Card like I did, or report online."
A spokesman for the National Pharmacy Association, which represents community pharmacists, said that it welcomed the campaign.
"Yellow Cards certainly used to be the domain of just the healthcare professional, but I think the MHRA have now made them much easier to use for the public.
"We wholeheartedly support this and are encouraging our members to promote the scheme."