The Muslims fast every year during the month of Ramadan. A fasting day can last 12-17 h. The effects of fasting on serum lithium levels and the mood changes in patients suffering from bipolar affective disorder during Ramadan are not well studied. We aimed to compare the serum lithium levels, side effects, toxicity and mental state in patients suffering from bipolar affective disorder and on prophylactic lithium therapy before, during and after Ramadan. Sixty-two patients meeting the International Classification of Diseases, Tenth Revision, Research Diagnostic Criteria of bipolar affective disorder receiving lithium treatment for prophylaxis were recruited in a tertiary care teaching hospital in Peshawar, Pakistan. Serum lithium, electrolytes, Hamilton Depression Rating Scale (HDRS) and Young Mania Rating Scale (YMRS) were assessed at three points, 1 week before Ramadan, midRamadan and 1 week after Ramadan. The side effects and toxicity were measured by a symptoms and signs checklist. There was no significant difference in mean serum lithium levels at three time points (preRamadan=0.45±0.21, midRamadan=0.51±0.20 and postRamadan=0.44±0.23 milli equivalents/litre, P=0.116). The scores on HDRS and YMRS showed significant decrease during Ramadan (F=34.12, P=0.00, for HDRS and F=15.6, P=0.000 for YMRS). The side effects and toxicity also did not differ significantly at three points. In conclusion, the patients who have stable mental state and lithium levels before Ramadan can be maintained on lithium during Ramadan. Fasting in an average temperature of 28°C for up to 12 h per day did not result in elevated serum lithium levels or more side effects and did not have adverse effects on mental state of patients suffering from bipolar affective disorder.