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Last week, crippling pelvic pain landed me in hospital. A doctor prescribed a course of antibiotics lasting just three days. I couldn鈥檛 help worrying that it wasn鈥檛 long enough. What if my infection hadn鈥檛 cleared by then? What if some of the harmful bacteria survived the treatment, and my body incubated the deadly, drug-resistant superbugs we have all been warned about?

Really, I had nothing to worry about: growing evidence suggests that short courses of antibiotics can be just as effective at killing bacteria as longer ones. And they don鈥檛 increase the risk of antibiotic resistance, at least for the common infections that most people receive antibiotics for.

In fact, it is the longer courses that cause problems. In 2010, an analysis of 24 studies, which included thousands of patients with respiratory and urinary tract infections, found that people on longer courses of antibiotics were .

That鈥檚 because most of the drug-resistant infections we are worried about stem from 鈥渇riendly鈥� bacteria that normally live in peace on and in our bodies. When we take antibiotics, we wipe out friendly gut bacteria along with the harmful ones. This disruption to the ecosystem can allow once-friendly bacteria from elsewhere in the body to colonise, where they can turn hostile and cause problems. Because these species weren鈥檛 killed by the antibiotic, any infections they cause are resistant to it. And this is more likely to happen with a longer course of treatment.

So why do many prescriptions tend to last for one or two weeks? When at Brighton and Sussex Medical School in the UK tried to find the origin of antibiotic prescription lengths, he struggled. 鈥淚t appeared that people working in the 1950s arrived at these, probably because they were worried that people would otherwise skimp on treatment, or because they were afraid of resistance,鈥� he says.

鈥淣o medical basis鈥�

Antibiotics are often prescribed in multiples of five or seven days. This is probably because these numbers correspond to the number of fingers on a hand and the number of days in a week, but there鈥檚 no medical basis, says Llewelyn, who co-authored a letter on the subject in the BMJ today. In fact, it might be a better idea to stop taking antibiotics once you feel better, and symptoms are resolved, he says.

The notion of cutting down overly long antibiotic courses isn鈥檛 new. The UK health agency Public Health England and the US Centers for Disease Control have both changed their guidance on antibiotic prescribing in recent years, based on an understanding of the evidence. Both organisations have ditched their 鈥渃omplete the course鈥� messages for ones that focus on following a doctor鈥檚 advice.

But the World Health Organization continues to promote the idea that completing a course of antibiotics is a . It鈥檚 an outdated position, and it鈥檚 time the WHO modified its stance.

In the meantime, people who are diagnosed with bacterial infections can discuss the course of treatment with their doctors 鈥� who also might not be up to date with the latest evidence. Three days of antibiotics worked in my case, and I鈥檓 pleased my doctor didn鈥檛 ladle me with extra medication when I didn鈥檛 need it.

Read more: Antibiotics apocalypse: Tales from fighters on the front line; Antibiotic resistance will hit a terrible tipping point in 2017