It is always worth checking to see whether any systematic reviews which have a concept in common with your search question have published their search strategy. If they have then this will act as a useful starting point for you to use for your search. Remember it is not necessary for all the concepts of the systematic review to be the same as you should be able to isolate the specific lines of the search strategy relevant to you. So for example if I am undertaking a systematic review on the 'effectiveness of phototherapy for neonatal jaundice' I may find that part of a search strategy for a published review on 'Phototherapy for treating pressure ulcers' is very relevant for me. I will then need to either create a search strategy element for my other concept of 'neonatal jaundice' or locate another systematic review which may have already created a search strategy including an element for this concept, e.g. one on 'Early intravenous nutrition for the prevention of neonatal jaundice'.
Below is an excerpt from a published Cochrane systematic review on 'Phototherapy for treating pressure ulcers' showing the search lines for the phototherapy concept (optimised for Ovid Medline) which could then be used in a different search and combined with other concepts.
Chen C, Hou WH, Chan ESY, Yeh ML, Lo HLD. Phototherapy for treating pressure ulcers. Cochrane Database of Systematic Reviews 2014, Issue 7. Art. No.: CD009224. DOI: 10.1002/14651858.CD009224.pub2.
Systematic reviews in the Cochrane Library should all publish their search strategies and in many cases will show the search strategy they used for each database they searched (optimised for each database) - this normally appears in the Appendix of the full text for a Cochrane review.
The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) checklist states that Systematic Reviews should "present full electronic search strategy for at least one database, including any limits used, such that it could be repeated". However, unfortunately you may find that older systematic reviews and even some current ones may not do this. Note too that in some cases the full search strategy may appear as a supporting document on a journal site as opposed to forming part of the pdf of the article.
Pay attention when looking to use a published search strategy to both the database and the platform it is hosted on and for which the search strategy has been optimised. If you are searching the same database on the same platform then you should simply be able to copy and paste the search strategy in to generate results. If you are uncertain about what the lines of the search strategy mean and what they are searching, e.g. exp, adj or .tw then see the information on the Advanced search techniques tab.
Remember when using a search strategy from another systematic review that you should assess it for quality rather than simply copying it in. Are there any other search terms that should be included, have they located all appropriate subject headings, and so on? See the information about PRESS: Peer Review of Electronic Search Strategies on the Advanced search techniques tab. The age of the source systematic review is also important as subject headings change to reflect changes in medical science. As such if you are working in an area of rapid development and the strategy is a few years old you may wish to just use it as a starting point rather than directly copying.
Finally if you do use all or part of another's search strategy then do remember to acknowledge the source.