"After careful review of the manufacturing system, manufacturing errors by the makers of Nurofen Plus or Seroquel XL are not thought to be part of the cause at this stage," the company said in a statement on Thursday. "We are taking this matter very seriously and are working closely with the regulatory authority, the MHRA, and pharmacies. "The MHRA are investigating the issue, and have considerable law enforcement powers. People who accidentally take Seroquel may experience sleepiness and are advised not to drive or operate any tools or machinery until they know how the tablets have affected them. A spokeswoman for AstraZeneca said people who have mistakenly taken the drug should contact their GP and take the medicine back to the pharmacy where they purchased it.