You likely are not going to like my answer.
I was wondering, is there ever an excuse to leave an internship early
Yes. There are plenty of reasons to do so, for example:
- You are not being assigned work or are completely neglected
- The work you are doing is unrelated to your career aspirations
- You are being forced to work unreasonable hours
- Some sort of family emergency
However. You must have conversations with your employer for your internship about these questions. An unbelievable amount of interns have problems with this. You must be assertive about your experience, needs, and wants with your manager(s).
For many interns, rather than a direct manager, there is a "mentor" type figure - perhaps your day-to-day manager or even an officially assigned mentor. If this is the case, replace manager with mentor in the following.
Your manager is not a psychic. They don't read your mind. You could be hating your job and your manager might assume things are going great - after all, you never talk to them about any concerns you have! Must be going great.
Some questions you need to ask yourself:
- Have I talked with my manager about feeling included in lunchtime conversations?
- Have I talked with my manager about having to work many more hours than I expect?
- Have I talked with my manager about the experience I am having as an intern?
- Have I talked with my manager about missing friends/family?
- Have I talked with my manager to discuss how much extra work I'm putting in?
Until you do these, you do not have a legitimate (see note below) excuse for leaving early. If you have those conversations and your manager goes, "I don't care" only then do you have a good reason to leave an internship early. But not even bothering to address the problems with your manager is a really, really, really poor reason to leave a job.
In signing up for the internship, you know the timeframe, you know the location. Just because your feelings change on this is a poor reason to leave a job - now, perhaps your conversations above with your manager indicate they would prefer you to not stick it out in an environment you don't like, at which case you have a much better situation. But saying "this sucks, I'm out" (which is effectively what you are suggesting) is really not good.
or would it pose too much of a risk to my future chances of employment?
Honestly, no, probably not. With that company? Yeah, but, unless you go into length about why you left an internship most employers will look at your resume/CV and see an internship and think to themselves, "oh, this person was a student. internships are always short!" and not think "oh, this person was going to intern for 12 months but only interned 6 months, must be suspicious."
note: you can probably legally leave the job nearly any time you want for any reason. You may have university obligations which cause a lack of credit, however.
This answer might also be valuable to you for information.