The short answer

Yes, in most cases: take your usual tablets with water unless whoever requested the test has told you otherwise. An NHS hospital leaflet puts it plainly: if you are unsure, continue your medicine as prescribed until you have checked with your doctor, because the result can often be read with the medicine taken into account. That holds before a fasting test too.

The exceptions are few and specific: medicines that lower blood sugar, a handful of doses best taken after the test, and supplements that can distort the measurement itself. The sections below show how to spot the cases where the answer changes.

Never stop a treatment on your own

Do not stop a treatment to make a number look better. The NHS notes that for a short time before some tests you may be asked to stop certain medicines, such as anticoagulants, but that is a decision for the GP, nurse or specialist, who should tell you if it applies. Interrupting an anticoagulant, a blood pressure tablet, an epilepsy medicine or a thyroid replacement on your own can carry a real risk, while the number itself only carried an inconvenience.

Taking tablets with a glass of water does not break a fast either: plain water is allowed throughout, right up to the test.

Diabetes medicines and a fasting test

This is the exception that matters most for your safety. Insulin and some diabetes tablets, such as gliclazide and glimepiride, can make blood sugar go too low, and the NHS lists missing or delaying meals among the situations that make a low more likely. A fasting test is exactly that.

East Kent Hospitals’ leaflet asks anyone with diabetes to speak to their doctor before starting a fast for any test. Ask in advance what to do that morning: whether to take, reduce or delay the dose. Book the earliest appointment you can, bring something to eat for straight after the test, and tell the staff at once if you feel shaky, sweaty or dizzy while you wait.

Levothyroxine and other timed doses

A few treatments are not stopped but timed, so that the sample shows your usual level rather than a peak. Levothyroxine is the common example: after a dose, blood levels of thyroxine (T4) and free T4 rise by about 15%, peaking around four hours later. A free T4 taken in that window can read higher than your usual level.

That is why you may be asked to take it after the test on the morning of a thyroid check. Follow the instruction you were given, and if there is none, ask. Either way the dose is taken, just later in the morning. The British Thyroid Foundation also asks you to tell the person taking your blood about any thyroid medication, especially if you have been taking too much or too little.

The same logic applies whenever a test measures the level of a drug itself rather than its effect. The request should say when to take the dose relative to the test; if yours does not, ask before swallowing it.

Biotin, iron and other supplements

Supplements are not neutral just because you bought them at the chemist. Biotin is the clearest case. A 2023 safety letter, agreed with the MHRA, warns that biotin can interfere with thyroid tests that rely on a biotin-based method, giving falsely high or falsely low results, and that multivitamins and supplements for hair, skin and nails can contain it.

In a study of six healthy adults taking 10 mg of biotin a day for one week, 9 of the 23 biotin-based tests checked gave potentially misleading results, some falsely high and some falsely low. The tests studied included thyroid hormones, parathyroid hormone and vitamin D.

Iron is the other common one. If iron studies are on the form, Lab Tests Online UK advises no iron tablets for 24 hours before the test, because iron is absorbed quickly and can make blood levels look falsely high; you may also be asked to fast for 12 hours.

What medications affect blood tests?

Many can, in different ways, which is why the list you bring matters more than any table. For thyroid tests alone, the British Thyroid Foundation names the contraceptive pill, steroid hormones, epilepsy medicines, anti-inflammatory drugs, lithium and amiodarone, plus supplements containing iodine or biotin.

From this page alone: diabetes medicines change blood sugar itself, levothyroxine briefly lifts free T4 after a dose, and biotin can disturb the measuring method. None of these makes the test useless; they make the context essential. Laboratory results drive about 70% of clinical decisions, so a result read without knowing what you take can steer a decision the wrong way.

What to tell the person taking your blood

Bring a list, or your repeat prescription slip. NHS hospital guidance asks you to tell your doctor about herbal remedies, vitamins and supplements as well as prescribed medicines, because they can also affect results.

If you stopped something on your own before realising, say so rather than hiding it. A result read as though you had taken your treatment, when you had not, is worse than a result with a note on it.

And if nobody gave you any instructions at all, you are not alone: in one survey of outpatients arriving for blood tests, more than half had received none. Asking is normal.

  1. Name and doseEvery prescription, as it appears on the box or your repeat prescription slip.
  2. Time of the last doseEspecially anything you took on the morning of the test.
  3. Supplements countVitamins, biotin, iron and herbal remedies belong on the list.
  4. Anything pausedSay if you stopped or delayed something, and on whose advice.