Nitrofurantoin (Macrobid): cystitis antibiotic and cautions

Nitrofurantoin is prescribed for lower urinary tract infections such as cystitis, and sometimes to prevent them. The NHS explains that the body rapidly filters it into the urine, so a high concentration builds up where the bacteria are. The same property is why it is not useful for infections elsewhere in the body.

According to the NHS it kills bacteria by entering their cells and damaging their genetic material. It comes as capsules, tablets and an oral liquid, and Macrobid is the brand mentioned.

The NHS gives a treatment dose for adults of 100 mg as slow-release capsules twice a day, or 50 mg to 100 mg of standard tablets four times a day. For prevention it gives 50 mg to 100 mg once a day at night. Swallow capsules and tablets whole without chewing, with or after a meal or snack.

Treatment of an active infection is usually 3 to 7 days. Prevention can run for several months, and some people take it around surgery, from the day of the operation to 3 days after. Always follow your label.

• Missed dose: take it when you remember unless the next is near, and skip it then. Never double up.

• One extra dose usually causes no harm; call NHS 111 if you took more than prescribed and have concerning symptoms.

• Finish the course even if you feel better.

Most adults and children can take nitrofurantoin. The NHS lists porphyria, G6PD deficiency, severe kidney disease, diabetes, lung conditions, liver problems, conditions causing severe weakness, anaemia or vitamin B deficiency and earlier allergic reactions as reasons to tell the prescriber.

The kidney point is practical: because the medicine relies on the kidneys to reach the urine, serious kidney disease changes whether it can be used.

Dark yellow or brown urine is common and harmless, returning to usual colour after the course. Other common effects are nausea, vomiting, diarrhoea, loss of appetite, headaches, and dizziness or sleepiness, in which case you should not drive or use machinery.

• Call NHS 111 for a new cough, coughing blood or mucus, or shortness of breath, which can point to a lung reaction.

• Call NHS 111 for yellow skin or eyes, or dark urine with pale stools.

• Call NHS 111 for unexplained bruising, bleeding, sore throat, fever or severe headaches.

• Call 999 for chest pain, breathing difficulty, pins and needles, numbness or weakness, or signs of anaphylaxis.

The NHS says nitrofurantoin may be used in pregnancy but is generally avoided towards the end, because there is a small chance of problems with the baby's red blood cells; doctors typically choose another antibiotic beyond 28 weeks.

It is generally acceptable while nursing a healthy baby, in short courses. It should be avoided if the baby has jaundice, raised bilirubin or G6PD deficiency. There is no link with reduced fertility.

The NHS names antacids containing magnesium, over-the-counter cystitis remedies and quinolone antibiotics such as ciprofloxacin, levofloxacin and ofloxacin. Oral typhoid vaccines may not work properly while you take it.

If you get regular UTIs the NHS suggests reducing alcohol, since it irritates the bladder. Drinking plenty, hygiene habits, cotton underwear and emptying the bladder fully after sex are listed as ways to reduce recurrence.