Drugs last longer at different times of day

James Cheeseman (left) and Dr Guy Warman studied the drug rocuronium. Picture / Greg Bowker

New research shows the length of time a drug lasts is influenced by the time of day it is administered.

A study by the Auckland Medical School into muscle relaxant rocuronium shows the drug lasts up to 15 minutes longer when given in the morning than in the afternoon. Rocuronium is used in combination with other drugs in general anaesthestic surgery.

Department of anaesthesiology lecturer Guy Warman said the finding added a new factor in determining drug dosage. "Time of day is not usually considered when deciding a drug dose."

Dr Warman heads a number of research projects into the body clock, which if better understood, has implications in the treatment of many conditions, including sleep disorders.

The rocuronium study was prompted by observations from anaesthetists on the varying times the same dosage lasts when all other factors, such as weight and gender, were taken into account. Dr Warman said the findings have importance in helping minimise complications, and in better predicting the time a drug will last.

Principal researcher James Cheeseman said it has implications, too, in the number of surgeries that can take place in an operating theatre. If a patient has not completely recovered from rocuronium, they cannot be ushered out of the theatre. "If someone doesn't use so much of a drug, then that's got to be a good thing."

The study, which is nearly complete, looked at 60 patients undergoing general anaesthetic surgery at Auckland City Hospital at different times of the day. A muscle relaxant was chosen because the drug's effects were easy to observe, said Mr Cheeseman.

He found that the patient's weight remained the biggest factor in determining dosage, but time of day was a more subtle factor.

Mr Cheeseman said it is not known why the results were so clearcut. One theory is that bloodflow to the liver, which breaks down the drug, varies according to the body's daily rhythm.