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For admissions to medical school, certainly try to get as much as you can, in a range of settings. Try to also get some volunteering experience.
as soon as you can because you will want to do as much as you can. commitment is important here. for example, a year's worth of volunteering is more valuable than a month or two here and there.
Start early. Shadowing in a hosptial is often alot more challenging that it is worth if you are under the age of majority for your state. Private practices allow you the most time with the person you are shadowing. Spending less than half your time in your favorite speciality is a good idea too. Shadow all levels of the health care ladder to show that you really are well rounded. Example: nurses, nurse practioners, phyisical therapists, occupational therapists, physican's aides, the office people, dentists, physicans, chiropractors. anyone you can think of.