Medicines and kidney impairment: what changes and why

Many medicines, or the products they are broken down into, leave the body through the kidneys. If filtration falls, the medicine can accumulate, and a dose that was right a year ago may now be too high. Some medicines also affect the kidneys directly, by altering blood flow to them or by irritating kidney tissue.

The NHS notes that kidney function affects how medicines are processed and cleared, and that certain long-term medicines, including lithium, omeprazole and anti-inflammatory painkillers, can affect kidney function, so people taking them have regular tests. That is the logic behind most kidney-related medicine advice: check the filter, then fit the medicine to it.

Two results describe the kidneys. The estimated glomerular filtration rate, or eGFR, reflects filtering capacity and is divided into stages G1 to G5, where stage 5 means almost all function has been lost. The albumin to creatinine ratio, or ACR, is a urine test for protein, graded A1 to A3. Together they stage CKD and help clinicians choose treatment and monitoring schedules.

The Specialist Pharmacy Service stresses that the different methods of measuring kidney function have limits and are not interchangeable. Prescribers therefore pay attention to which measurement a dosing recommendation was written for, and to whether the person is very small, very large or acutely unwell, in which case a single number may mislead.

The NHS treatment page for CKD is explicit: avoid over-the-counter NSAIDs such as ibuprofen unless a doctor has advised otherwise, because they can harm the kidneys. NHS Inform adds that people with kidney function below 50 per cent should avoid NSAIDs including ibuprofen, aspirin taken as a painkiller and diclofenac, which can worsen damaged kidneys. It makes an exception for low-dose aspirin prescribed to prevent vascular disease.

NHS Inform recommends paracetamol for headaches in CKD but advises avoiding soluble or effervescent forms because of the sodium they contain. Kidney Care UK also advises caution with combination products containing paracetamol, such as co-codamol. For muscle and joint pain NHS Inform suggests topical preparations are safer than tablets.

People with CKD often take ACE inhibitors or angiotensin receptor blockers, which protect the kidneys and lower blood pressure. NHS Inform explains that both can reduce kidney function in some people and raise potassium, so blood tests are needed after starting. Specialist pharmacy monitoring guidance notes that some rise in creatinine and potassium is expected after starting, and that a high potassium level in CKD can mean stopping the medicine and seeking specialist advice.

The NHS also lists statins to lower cardiovascular risk, SGLT2 inhibitors such as dapagliflozin for blood sugar and reduced kidney workload, and diuretics for fluid build-up. It advises avoiding potassium supplements and certain blood pressure or heart failure medicines when potassium is high. All of these need monitoring, which is why attending scheduled blood tests matters.

The NHS says some painkillers and nutritional supplements can affect kidney function or interfere with prescriptions, and recommends professional advice before using them. NHS Inform warns that some cold remedies are unsuitable for those with high blood pressure, since decongestants can raise it, and says to check labels carefully.

Kidney Care UK advises avoiding effervescent tablets because of their sodium content, and returning out-of-date medicines to a pharmacy rather than flushing them. It recommends telling every professional about the kidney condition, since not all prescribers or dispensers will see the full record.

The Specialist Pharmacy Service describes routine reviews in CKD that check for changes in kidney function, consider whether evidence-based therapies such as statins, antiplatelet medicines or SGLT2 inhibitors are appropriate, and look for interactions with over-the-counter and complementary products. Records should be kept up to date across care settings so that a hospital doctor and a GP are working from the same list.

A review also happens when something changes: an infection, a hospital stay, a new prescription or a dehydrating illness. Guidance advises reviewing medicines in people with acute kidney injury and in those at increased risk during intercurrent illness.

Sudden reduction in urine, marked thirst, nausea, leg or foot swelling, unusual tiredness, confusion or breathlessness can signal acute kidney problems. The NHS advises urgent care or NHS 111 for these symptoms. Do not wait for the next routine blood test if you are unwell and taking medicines that affect the kidneys.