Desmopressin

Desmopressin copies vasopressin, which the pituitary gland releases whenever the body must conserve water. As a result the kidneys take back more water, so there is less urine, and it is more concentrated. It lasts longer than natural vasopressin, and the dose is tailored to the individual. A strength of 200 micrograms is the same as 0.2 mg, the size of the UK tablet.

In cranial diabetes insipidus too little vasopressin is produced or released, so a person produces large volumes of dilute urine and is constantly thirsty. The 0.2 mg tablet is licensed for this, and for the same two symptoms (polyuria and polydipsia) after pituitary surgery. A common start is 0.1 mg three times a day. Most patients end up needing 0.1 to 0.2 mg three times daily, within a total of 0.2 to 1.2 mg a day, and after surgery urine osmolality guides the adjustment. Before the diagnosis is confirmed, compulsive water drinking and alcohol misuse need to be ruled out.

A second UK licence, held by Desmotabs 0.2 mg, applies to primary nocturnal enuresis between the ages of 5 and 65. At bedtime 0.2 mg is taken; if needed the dose goes to 0.4 mg, and after three months a break of at least a week shows whether treatment is still required. Treatment is for people with normal blood pressure, and it should not be started after the age of 65. Fluids must be limited starting one hour before the bedtime dose and continuing until morning, which is no less than eight hours after the tablet. Excess drinking after a dose is the usual way water retention develops, which is why the rule is non-negotiable. Vomiting or diarrhoea means bedwetting treatment stops for the time being, and swimmers should watch how much water they swallow.

The tablets are sold in 30, 60, 90, 120 or 180. The tablet itself is the same for both purposes; only the timetable differs, with up to three doses a day in diabetes insipidus and a single bedtime dose for enuresis. Imported cartons can look unlike UK ones, so keeping the box for a prescriber or pharmacist to inspect is wise. Desmopressin sublingual tablets in the UK have different strengths, so they are not interchangeable with these on a like-for-like basis.

Heart failure and any condition treated with diuretics rule out desmopressin. Reduced kidney function, heart or circulatory disease, cystic fibrosis and a raised pressure inside the skull all call for caution. Tricyclic and SSRI antidepressants, chlorpromazine, carbamazepine, anti-inflammatory painkillers and loperamide may increase the likelihood of low sodium or alter the response, and a fatty meal changes absorption. Pregnancy calls for caution and blood pressure monitoring, according to the SmPC.

Prescribers tend to ask how much is drunk and how much urine is passed by day and night. A sodium test before starting is common, with another after about a week, older people being the main concern. Writing down fluid intake for the first few days is useful. Bedwetting treatment is rarely lifelong, and the planned break is there to see whether the problem has resolved of its own accord. If vomiting or diarrhoea strikes, bedwetting treatment is paused until fluid balance is normal; people with diabetes insipidus should get urgent medical advice instead of simply stopping.

Headache, abdominal pain, nausea and allergic reactions have been reported, and children may become emotionally unsettled or aggressive. Hyponatraemia, a shortfall of sodium in the blood, is the risk to watch for. Headache, nausea, confusion and weight gain are early signs, and seizures can follow in severe cases, which need urgent help. Great drowsiness or confusion after a dose is an emergency. Where Desmopressin is used, the prescriber matches dose and fluid advice to the reason for treatment and arranges sodium checks when needed.