I got an offer letter from an employer that said my employement is contingent on a successful background check. There is no mention of drug screening anywhere. Do employers have to explicitly state that there will be drug screening or is this already implied by the mentioning of "background checks."
Short answer: It's possible.
The thing to take into consideration is the type of job and industry you are in. If you are taking a job where having no drugs (such as heavy machinery operator and many government jobs), then there is likely an implication that yes, there would be a drug test. Other industries such as retail normally would have no such requirement.
Otherwise, well it's up to the company. There is no universal standard definition of "background checks", it can mean whatever the company decides it means. So my advice is, if you think that having drugs in your system may adversely affect your chances of passing background checks, then you should get off them as soon as possible to minimise the risk of losing the job.
Do employers have to explicitly state that there will be drug screening or is this already implied by the mentioning of "background checks."
I don't believe in the US there is a federal law requiring that they warn you ahead of time that the "background check" will include a drug test. And each state has their own laws. See: http://www.nolo.com/legal-encyclopedia/free-books/employee-rights-book/chapter5-3.html
That said, every time I have encountered a situation where a drug test was to be performed, I was actually notified ahead of time.
If this is a situation that could be dicey for you, you should prepare as if you will be drug tested - whether it is mentioned ahead of time or not.
"Background check" can include lots of things, but not necessarily any particular thing. I've had jobs where it has meant a criminal background check, a credit check, or just a reference check, without including a drug screening. However, as already indicated, that doesn't mean that a drug test isn't a possibility.
To be really sure, you need to check with the potential employer. Of course, just coming out and saying something like "Hey, does this background check include a drug test?" could be a red flag to the employer. However, if you phrase the question correctly, you may able to find out what is entailed in the background check - a query along the lines of "I want to facilitate this process as much as possible, what do I need to do to complete my background check?" may allow you to find out if a drug test is required without raising suspicions.
is this already implied by the mentioning of "background checks."
Not usually, but don't take any chances, stop smoking any dodgy herbiage or whatever else it is you're worried about for a while.