The incident happened at Rowlands Pharmacy yesterday at around 6am in Blanche Street, Ashton.
The pharmacy was broken into and a supply of prescription drug Tramadol was taken.
A Lancashire Police spokesman said: "We would like to remind the public not to accept or take any medication from other members of the public.
"Medication that is not prescribed to you can have serious and/or life changing effects on you.
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"If you have any information regarding this incident, please ring 101 quoting incident number LC-20181029-0148."