A pill to help people stop smoking is under the spotlight amid reports that it can generate suicidal feelings.
Some 44% quit smoking in trials of the drug
The UK's medicines watchdog, the MHRA, is to monitor the situation "closely" after 839 reports of adverse reactions to Champix, licensed last year.
Nearly 50 people reported depression - mainly patients with a previous psychiatric history. There were another 16 reports of suicidal thoughts.
The US regulatory body has already expressed concerns about the drug.
Pfizer's drug Champix, also known as Varenicline, has been available for about a year.
It is unusual as it both stimulates and blocks specific nicotinic receptors in the brain.
It is thought that by stimulating the receptor it mimics the effects of nicotine to reduce cravings.
At the same time, it partially blocks the receptor preventing nicotine from binding to it, resulting in a weaker response in people who give in to temptation and have a cigarette.
Trials suggest some 44% of smokers give up after taking the drug twice a day for 12 weeks, compared with 18% of those given a placebo and 30% of those taking another major anti-smoking drug, bupropion.
Nausea was already known to be a side-effect, and 183 of the reactions which had been reported to the Medicines and Healthcare products Regulatory Agency by late September involved feeling sick.
There were 52 reports of abnormal dreams, 49 of dizziness, 37 of fatigue, 82 of headaches, 21 of drowsiness and 67 of vomiting.
The majority of the reports of depression came from patients with a previous psychiatric history.
In the UK, between 15,000 and 20,000 people are using the drug.
Last week, the US Food and Drug Administration announced it was investigating the drug after receiving similar reports.
The European Medicines Agency said they looked at some of these concerns in July, and had decided no action was needed. Current warnings were adequate, they concluded.
A statement from Pfizer said: "There is no scientific evidence establishing a causal relationship between varenicline and the post-marketing report events.
"Quitting smoking, with or without treatment, is associated with nicotine withdrawal symptoms and has also been associated with the exacerbation of underlying psychiatric illness."
1. Nicotine from a cigarette stimulates the release of dopamine - a substance produced by the body which triggers feelings of pleasure
2. When a smoker quits, the lack of nicotine leads to reduced levels of dopamine, causing feelings of craving and withdrawal
3. Varenicline both blocks the nicotine receptors (reducing the addictive power of the drug) and triggers moderate dopamine release to alleviate withdrawal symptoms