Balwinder Kaur Bolla, a specialist clinical pharmacist for the care of complex needs, considers the effect that smoking and caffeine can have on lithium levels

Mr GW, a 67-year-old male, was admitted to hospital with tremor and confusion which had lasted for several days. His past medical history includes bipolar disorder, for which he takes Priadel MR tablets 600mg, one in the morning. He was found to have lithium toxicity (plasma level 2.05mmol/L) but there was no evidence or suspicion of overdose.

Renal function on admission was 57ml/min but, compared to his baseline of 70ml/min, this could not be held accountable for the raised lithium level. He had been stable on his lithium dose for several years. He had also been smoking around 20 cigarettes a day for the past 30 years but says he had tried to stop smoking abruptly the week before admission.

This prompted an investigation into whether smoking has an effect on lithium levels. During the hospital stay, Mr GW was also found to have sub-clinical hypothyroidism, which was attributed to the long-term effects of lithium, and he was initiated on levothyroxine 25mcg daily.

Effect of smoking?

Lithium has many documented interactions and warnings but little information is readily available about the effects of smoking. Studies show that smokers metabolise caffeine almost twice as fast as nonsmokers and caffeine affects lithium excretion. The SPC for Priadel MR tablets does mention that lithium renal clearance is increased by caffeine.

Patients on lithium are generally advised to keep caffeine intake the same but information on the effects of smoking is not highlighted and is often neglected in clinical and prescribing decisions.

It would appear that smoking cessation decreased Mr GW’s lithium levels. However the increased clearance of caffeine by smokers may contribute to the higher caffeine consumption reported in this group.

Many smokers cannot tolerate their usual caffeine consumption when they give up smoking as they are not accustomed to the higher plasma levels, so they cut down on their caffeine intake.

This reduces lithium excretion, posing a risk of accumulation and toxicity, which is what happened in Mr GW’s case. He had been on lithium for such a long time that when he stopped smoking, he subconsciously reduced his caffeine intake quite significantly.

What this case shows is that, while existing initiatives assess and monitor the interactions of drugs among themselves, it is also prudent to look at how lifestyle can interact with medication regimens.