Gentamicin: an injected hospital antibiotic, its monitoring and risks

Gentamicin is an injectable antibiotic. It belongs to the aminoglycoside group, which the NHS describes as antibiotics that tend to be used only in hospital to treat very serious illnesses such as sepsis. The NHS also notes that they are usually given by injection, though some are available as drops for ear or eye infections.

The leaflet explains that the right dose depends on weight, the type of infection and kidney function, and the NHS warns of serious side effects including hearing loss and kidney damage. That is why monitoring is built into its use.

The brand name Cidomycin appears on some UK injectable products, and unbranded gentamicin products are also listed on the electronic medicines compendium. It is given only by healthcare staff.

The leaflet for gentamicin injection lists infections of the urinary tract including the kidneys and bladder, the chest including the lungs, the abdomen including the gut, the brain and spinal cord, and the blood, sometimes called bacteraemia or septicaemia. It also lists infections in newborn babies.

As with other hospital antibiotics, the choice is often made before the exact bacterium is known. The NHS sepsis page explains that treatment for sepsis starts with a general antibiotic and may then be changed to a more specific one once the cause is confirmed. Gentamicin is therefore one of several options that may appear on a drug chart in the first hours of a serious infection.

Not every infection suits it. The leaflet says gentamicin should not be used by anyone with myasthenia gravis, a condition that causes muscle weakness, and the NICE guideline for neutropenic sepsis advises against starting with an aminoglycoside unless there are patient-specific or local reasons.

The leaflet says the usual adult daily dose is 3 to 5 mg for every kilogram of body weight, given preferably as one single dose, though it may be split into two or three. Doses for children and babies are calculated by weight and are different for premature babies and newborns. Those who have kidney problems may be given a lower dose or a longer gap between doses.

Blood samples are taken so that the team can check the amount of gentamicin in the blood. The leaflet is clear that you should not receive gentamicin if the blood tests cannot be performed. Hearing, balance and kidney function may also be tested before, during and shortly after treatment.

The leaflet highlights people who need particularly close monitoring: those over 65, babies under 1 year old and newborns, people with impaired kidney function, people who are obese, and people with cystic fibrosis. If you fall into one of these groups you may find more blood tests are requested than you expected, and this is by design.

If you think you have missed a dose, tell the nurse or doctor. They hold the schedule, and the timing between doses matters because of how the drug is cleared by the kidneys.

The leaflet lists several serious reactions that should be reported to staff straight away. These include allergic reactions with a lumpy rash, swelling of the face or throat, or fainting; temporary or irreversible hearing loss or deafness; new difficulty moving; numbness, weakness and pain in the arms and legs; blood in the urine; and diarrhoea with or without blood or stomach cramps.

Other effects reported include vomiting, nausea, mouth ulcers, rash or itching, depression, hallucinations, confusion, tiredness, weakness and fits. Blood tests may show changes in blood cell counts, liver enzymes and a drop in magnesium with prolonged treatment. Very rarely, high doses over a long time have been associated with a kidney tubule problem called Fanconi-like syndrome.

The leaflet also describes severe skin reactions such as Stevens-Johnson syndrome and asks you to tell staff immediately if you have symptoms. Because hearing loss is listed as a possible effect, any change in hearing should be mentioned to the team during a course.

The leaflet says certain mitochondrial gene changes may increase the risk of hearing loss with aminoglycosides. If you or a maternal relative have had hearing loss after an antibiotic, say so before treatment, because your doctor may discuss genetic testing.

The leaflet lists blood thinners such as warfarin, water tablets (diuretics) such as furosemide, amphotericin B, cephalosporin antibiotics, ciclosporin, neostigmine and pyridostigmine, muscle relaxants used in operations, indometacin, bisphosphonates, cisplatin and botulinum toxin as medicines that may increase the chance of side effects. The point is that several of these also affect the kidneys or hearing, so using them together can add to the risk.

Tell staff about every medicine you take, including those bought over the counter and herbal products. Mention if you are pregnant or planning to be, because the leaflet says gentamicin is not recommended during pregnancy or breastfeeding. People with muscle weakness conditions, severe diarrhoea, cystic fibrosis or a history of severe skin reactions should be flagged to the team too.

Good hydration and attention to blood test results are the main protective steps, and they are managed by the ward team. What you can do is report symptoms early, using the leaflet list above: changes in hearing or balance, new numbness or weakness, blood in the urine or severe diarrhoea.

If you have already had gentamicin and developed hearing loss or balance problems, make sure this is recorded in your notes so that future prescribers can take it into account.

Gentamicin is given on NHS wards and in private hospitals in much the same way, with weight-based dosing and blood level monitoring. It is not something people usually buy or self-administer. If you are worried about hearing or kidney effects, ask the ward team what tests are planned and when the course is expected to finish.