If you donate blood will they check your blood for illegal drug use?

I'm not sure if this is the case in the US, but in Canada they only test for communicable diseases, low iron levels, etc. They do not care whether or not you have taken recreational drugs. Drug use does not affect the blood you are donating in any negative manner and there is no good reason for them to test for drug use at all.

It should be noted however, that if you have taken any drugs by needle, then you cannot give blood for a period of time (I think a year or two). They will ask you this beforehand along with anything else that may compromise your donation, so you don't need to worry about being tested for anything else.

Also, if for whatever reason you are embarrassed or unable to truthfully answer no to any of the questions (possibly a stern parent watching you give blood), you can continue with your donation, but choose to label the sample (in private) to be discarded. This is done by choosing one of two stickers, each with a bar code, so nobody in the testing area will know which you chose; it will be read later off-site.

So basically no, you do not need to worry about being tested for drugs.

However, you should be aware that people taking certain prescribed drugs cannot give blood to begin with - for example, those on antiretrovirals, immunosuppressives, or anticoagulants, to name but a few.