Many people assume that medicines sold over the counter are inherently safe. After all, if you can buy something in a supermarket or high street pharmacy, how dangerous can it really be?
The reality is more complicated. Several commonly used over-the-counter medicines carry a real risk of dependence, misuse or harm when taken in higher than recommended doses, for longer than needed, or for the wrong reasons. Here are five medicines it is worth knowing about.
1. Codeine-based painkillers
Codeine is an opioid medicine used to treat mild to moderate pain and, in some formulations, to suppress coughing. Over the counter, it is usually combined with either ibuprofen or paracetamol. Once swallowed, the body converts codeine into morphine, which produces its pain-relieving effects.
Common side effects include drowsiness, constipation, nausea and dizziness. At higher doses, codeine can slow breathing and impair coordination. Some people are particularly vulnerable. Ultra-rapid metabolisers carry a genetic variant that causes them to convert codeine into morphine much faster than usual. This trait is more common in people of North African, Middle Eastern and Oceanian backgrounds and can lead to dangerous side effects even at standard doses.
With repeated use, the body can also become tolerant to codeine, meaning the same dose no longer provides the same relief. This process, known as tolerance, occurs as the brain’s opioid receptors adapt to the drug. People may then increase their dose, raising the risk of physical dependence. Stopping suddenly can cause withdrawal symptoms such as anxiety, restlessness, sweating and sleep problems.
To reduce these risks, codeine should be used for the shortest time possible. In the UK, the Medicines and Healthcare products Regulatory Agency limits pack sizes to 32 tablets and advises non-prescription products should be used for no more than three days.
2. Decongestants
Decongestants are available as tablets containing pseudoephedrine or as nasal sprays and drops such as xylometazoline and oxymetazoline. Both work by narrowing blood vessels in the nasal passages, reducing swelling and mucus.
Overusing nasal sprays can lead to rebound congestion, known medically as rhinitis medicamentosa. Over time, the medication becomes less effective, a phenomenon called tachyphylaxis. This can trap people in a cycle of increasing use, worsening congestion and dependence.
Long-term overuse can damage the lining of the nose, causing dryness, nosebleeds and, in severe cases, perforation of the nasal septum. Many users also develop a psychological dependence on the spray. Most guidance recommends limiting use to three to five days.
Pseudoephedrine also has mild stimulant effects. Although evidence for improved athletic performance is mixed, its stimulant properties mean it appears on the list of substances banned in competition. It is also used illicitly to make methamphetamine, which is why strict sales controls remain in place following a 2016 review.
3. Sleeping tablets
Promethazine and diphenhydramine are sedating antihistamines sold as short-term sleep aids. Recent research has linked sedating antihistamines to rising numbers of deaths, prompting calls for a review of how they are supplied.
Promethazine can quickly lead to tolerance, meaning higher doses are needed to achieve the same effect. Some long-term users report severe rebound insomnia when they try to stop.
It is also used recreationally in “purple drank”, a mixture of cough syrup that contains promethazine and soft drinks. This combination can cause extreme sedation, slowed breathing and serious harm.
4. Cough syrups
Dextromethorphan (DXM) is a common cough suppressant. A 2021 review found it was the most frequently misused over-the-counter medicine studied. At high doses, it blocks NMDA receptors in the brain, which can cause dissociative effects similar to ketamine. While safe at recommended doses, its psychoactive effects have raised concerns about misuse.
5. Laxatives
Stimulant laxatives trigger the gut muscles to move stool along. They are often misused by people with eating disorders, athletes in weight-restricted sports, or those who believe daily bowel movements are essential. In reality, constipation is usually defined as fewer than three bowel movements a week.
Research shows stimulant laxatives do not prevent calorie absorption, despite common myths. Instead, misuse can cause dehydration, electrolyte imbalance and long-term damage to the gut, with serious effects on the heart and kidneys in severe cases. In 2020, the MHRA introduced new rules on pack sizes and warnings.
The common thread linking these medicines is not that they are inherently dangerous, but that their risks are often underestimated. Over-the-counter availability can create a false sense of security, particularly when medicines are bought online without professional advice. While regulators have taken steps, research suggests misuse persists. Over the counter does not always mean risk free, and better awareness could help keep these medicines useful rather than harmful.
