Fresh off the pages of Therapeutic Advances in Endocrinology and Metabolism comes a warning no self-respecting sweet tooth should ignore.

“Liquorice is not just a candy,” write a team of ten from Chicago. “Life-threatening complications can occur with excess use.” Hold on to your teabags. Liquorice – the Marmite of sweets – is about to become a lot more sinister.

In walk case reports by the dozen. “Lethal liquorice lollies” write Robles et al. in BMJ Case Reports. Japanese traditional liquorice-containing remedies cause potassium deficiency in a quarter of patients, according to a clinical trial. When a woman presented to Sheffield A&E with a “four-month history of headaches, nausea and dizziness” and severe hypertension – bingo – it was her liquorice tea, three cups a day. Chest pain, fatigue, inability to walk, muscle necrosis: all this in another patient, who in this case had been taking four glycyrrhizin tablets per day after catching a cold. Glycyrrhizin is the active ingredient in liquorice.

Again and again, patients present to the clinic with critically high blood pressure, organ impairment, and the inability to move their limbs. They stop overdosing on their liquorice tea or their herbal liquorice therapies and they recover rapidly.

Liquorice owes its taste and pharmacologic properties to glycyrrhizin, a combination of two sugars and a steroid. Chemical reactions in the stomach chop off one or both of the sugars, leaving the large steroid molecule – glycyrrhetic acid – or a metabolite called 3-MGA. It is up to 50 times as sweet as sucrose and it is used as a sweetener – for example, to mask the taste of bitter medicines. That’s why it has its own E-number (E958).

Monks introduced liquorice to Pontefract in 1562, and although the eponymous Pontefract Cake has been manufactured there for centuries, its use in food and drink traces its origins to well before the year zero. In addition to tea and candy, liquorice is found hard and soft drinks originating from Egypt to Bulgaria.

In this case, despite the attractive taste of glycyrrhizin, it is the metabolite 3-MGA that is problematic. Experiments with rat livers show that 3-MGA binds to a protein called 11-ß-hydroxysteroid dehydrogenase, which is responsible for converting the stress hormone cortisol into inert cortisone. Cortisol then mimics another hormone, aldosterone, which causes a deficiency in potassium.

This is a testament to the complexity of human physiology. Cortisol and aldosterone are similar chemically and are both produced in the adrenal glands. Aldosterone is the hormone which governs the salt/water balance, and although its purpose is to regulate blood pressure and the kidneys, potassium salt is essential to nerve function and muscle contraction. Too little of it causes high blood pressure, nausea, and all kinds of nerve problems.

So if you love liquorice, you need to be careful. Two thousand years of liquorice cultivation proves that a cup of tea and a handful of candy is fine. But if you’re having multiple cups a day, or if you’re taking high-dose herbal ‘medicines’, and if you start to feel dizzy or weak, lay off the liquorice. Stay safe out there.