Fact Check-Shell of gel capsule for prescription antibiotic flucloxacillin contains iron oxide, a common colouring agent used to protect the medicine inside

Social media users have shared videos showing black material in the gel capsule for the antibiotic flucloxacillin, which appears to be attracted to magnets when dissolved in water.

The capsule shell can contain the colour agent iron oxide, which is magnetic. Its use is heavily regulated and there is no evidence to suggest that it is dangerous.

Flucloxacillin, which is only available in the UK on prescription (here), is used to treat a range of infections including chest infections, such as pneumonia, as well as skin and wound infections.

Videos (here , here) shared online show an at-home science experiment. During the experiment, the capsule shell of the flucloxacillin is broken, and the medicine tipped out.

The shell – half of which is black in colour- is then dissolved in a glass of hot water.

When a magnet is swiped along the outside of the glass, a black blob is shown to follow the path of the magnet. In some of the videos, people can be heard suggesting the material is dangerous.

“Imagine you had a 5G electromagnetic pulse which you could manipulate this with...I wonder if it's possible? Well, I can certainly manipulate it with this,” says a man conducting the experiment in one video, as he moves the magnet across the glass (here).

An open access resource provides details of the inactive ingredients, otherwise known as excipients, used to package any medicine that is licensed in the UK. It can be seen

On the page for flucloxacillin here, the capsule shell is shown to contain iron oxide (E172).

It is a legal requirement of the UK Human Medicines Regulations 2012 that ingredients in a medicine are shown on the leaflet provided with each pack here.

Iron oxide, also known as ferric oxide, is used in foods, cosmetics and in the pharmaceutical industry (here).

“Iron oxide is used in medicines as a colour agent – it helps to distinguish medicines so that they don’t all look the same,” Hannah Batchelor, Professor in Pharmaceutics within the Strathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, explained to Reuters in an email.

“Iron oxide also makes the capsule shell opaque which reduces light ingress so helps the stability of the medicine.”

Colour additives allowed in UK medicines are determined by UK law, still linked to EU food legislation since Britain exited the bloc (here).

“The regulations for medicines in the UK are in line with Europe and the USA where every medicine needs to demonstrate a good safety profile, and the level of iron oxide will have been reviewed to ensure that it is below the daily limit,” Batchelor said.

“There is no reason to believe that this is unsafe.”

An acceptable daily intake (ADI) of 0-0.5 mg/kg bw/day of iron oxide was set by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) (here).

This means that for a 50 kg adult 25 mg could be ingested every day over an entire lifetime without any appreciable health risk.

The UK government says additives can only be used if they have been tested and proved to be safe for their intended use. It adds that the science on additives is strictly reviewed here.


Missing context. The shell of some medicine capsules can contain iron oxide, which is magnetic, as a colour agent. Iron oxide, which is also used in food, is heavily regulated and there is no evidence to suggest that it is dangerous.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts here.