Timothy L Brock/UnsplashLike all consumer products, sofas come with instructions for how best to use and maintain them.But it may surprise you that many furniture companies warn medicines can seep out of your sweat or body oils to damage your leather sofa.People who repair leather sofas warn about this too. They say this can lead to the leather fading, cracking or discolouring.So which medicines are they talking about? And how worried should you be about them damaging your furniture?Different drugs, different body partsWhen you take a drug – as a tablet, injection or other form – it is distributed to different parts of the body. And someone would likely have a detectable amount in most bodily fluids. This includes breast milk, tears, seminal fluid, saliva and sweat.How much of the drug reaches each part of the body, and where it ultimately accumulates, depends largely on the drug’s specific properties.Water-soluble drugs, such as aspirin and paracetamol, tend to accumulate in blood serum and other bodily fluids. Oil-soluble drugs, such as testosterone and ketamine, tend to accumulate in the organs and fat stores.When it comes to excretion, most of the drug will be filtered by the kidneys and removed from the body in urine. The amount of drug usually excreted in faeces is low and is mostly due to small quantities of drugs you take by mouth that didn’t end up getting absorbed into the body. For example, when someone eats a cannabis edible, such as a gummy, about 23% of the active psychoactive chemical (THC, tetrahydrocannabinol), is excreted in their faeces.But drugs can also be excreted through sweat and skin oil (sebum), when they diffuse from blood vessels into the sweat ducts and sebaceous glands. This is the type of damage you can expect from your medicines, according to one leather repairer. Mobile Leather Repairs/Screenshot So which drugs?Companies claim a long list of medicines can come through the skin and damage leather sofas. This includes drugs for blood/heart conditions and arthritis, chemotherapy and diabetes, and some vitamins, such as fish oil tablets.But are they correct?We’ve known drugs can be excreted in sweat since the 1970s. A study by Stanford University researchers found two antibiotics (sulfaguanidine and sulfadiazine), an anti-inflammatory called antipyrine and the fever-reducing drug aminopyrine in volunteers’ sweat samples.Other drug types, including common everyday legal and illegal drugs, have also been found in sweat. In a 2023 study, researchers at the University of the West of England found caffeine, nicotine, ketamine and cocaine in samples taken from volunteers’ arm and leg sweat.The chemotherapy drug cyclophosphamide has been found in cancer patients’ sweat in multiple studies. This includes in their underwear, pillow cases and bed sheets.We know how a drug is excreted in the skin depends on whether it is water- or fat-soluble. But there are also other factors involved.Smaller drugs, such as ibuprofen, generally cross tissues into sweat or skin oil more readily than larger ones, such as the peptide weight-loss drugs Ozempic and Mounjaro. The acidity of a drug plays an important role. Sweat has its own pH, so some drugs, such as amphetamines and opioids, can become trapped in sweat because of their charge, a phenomenon called pH or ion trapping. This means the drug concentration in sweat is higher than you would expect.Timing and conditions also play a role. Drug levels in sweat will be highest not long after taking it, and will vary depending on whether, and how much, you’re exercising, how much you are sweating, your skin’s oiliness, body and air temperature, and where on the body you are sweating.How about drug testing?Knowing that drugs can be excreted in body fluids opens up a range of possibilities for drug testing.European scientists have developed sensors to monitor sweat for the drug methotrexate, which is used to treat arthritis, psoriasis and cancer.Similarly, researchers in Switzerland have developed a sweat-based method for monitoring levels of the antibiotics flucloxacillin, imipenem and cefepime.Drugs are also present in high enough levels in saliva that scientists have examined whether saliva testing can replace blood tests to monitor people’s medicines when optimising their dose.So, back to drugs and sofasThe amount of drug excreted through the skin is so low as to be negligible to human health. It is generally in the order of nanograms of drug per millilitre of sweat. A nanogram is 0.000000001 grams. This is far too low to have an effect on anyone who comes into contact with it.The bigger issue is the potential staining, discolouration and damage that can theoretically occur in leather from prolonged contact with sweat, sebum and the drugs. But, as far as I can tell, the potential for drugs to damage leather sofas hasn’t been formally studied and published. So we’re going on anecdotal reports from retailers and sofa repairers.In fact, there may be more potential for your leather sofa to affect you, rather than the other way around. There is concern about the off-gassing of chemicals from the sofa’s leather that can potentially pose a health risk.If you are worried about drugs damaging your furniture, the best thing you can do is not sit on the sofa if you are particularly sweaty, or to protect the sofa with a blanket or towel. Follow the retailer’s instructions on how best to clean your sofa.If you’re worried you’re sweating more heavily than normal, see a qualified health professional, such as a doctor or pharmacist, for advice. Read more: 7 things you can do if you think you sweat too much Nial Wheate in the past has received funding from the ACT Cancer Council, Tenovus Scotland, Medical Research Scotland, Scottish Crucible, and the Scottish Universities Life Sciences Alliance. He is a Chartered Chemist and fellow of the Royal Australian Chemical Institute. Nial is the chief scientific officer of Vaihea Skincare LLC, a director of medical device company SetDose Pty Ltd and was previously a Standards Australia panel member for sunscreen agents. He is a member of the Expert Panel for the Haleon Pain Management Institute. Nial regularly consults to industry on issues to do with medicine risk assessments and medicines manufacturing, design and testing. Nial is often called as a expert witness in sports tribunals involving adverse analytical findings for elite athletes.