Tramadol is a synthetic opioid that has been available in the Netherlands since 1992 and is usually used as a centrally-acting analgesic when paracetamol or an NSAID provides insufficient relief. In the period 1 January 1992--30 November 2003, the Netherlands Pharmacovigilance Centre Lareb received 299 reports concerning 522 adverse drug reactions associated with the use of tramadol. Some of the frequently reported side effects with a high reporting odds ratio were nausea, constipation and withdrawal symptoms. These side effects are very similar to those of the other opioids due to the affinity of tramadol for the micro-opioid receptor. Because tramadol is often not recognised as an opioid, it is important that such opiate effects be recognised as an adverse drug reaction on time.