Can I safely lose ~5 pounds in a week and be able to race well?

I'm in the same position. Got shin splints running on hard tarmac in first week of vacation so stopped running for a week (but still swam); then drove for 3 straight days, with predictably bad diet. Stepped on the scales the morning after getting home (Sun morning) and I was 6 pounds up. Race coming this Saturday.

I did a medium run on Sunday, forcing myself to do 6x2min hard, 2min easy in the middle; felt very sluggish. Yesterday was a double (3m + light gym work am, 6m easy p.m.) and I was feeling more like a runner. I had lost 3 of those pounds already when I got on the scales this morning. The love handles have visibly thinned out in 48 hours.

Like most things, if you gain it fast you seem to lose it fast.

The best plan is to eat sensibly, train sensibly and be fresh for the race. If you've had time off and jump into a single super-session you'll just be sore. Get a bit of quality in every other day and you're saying to your body "sorry, not retired yet, back to work..." and it will respond appropriately.

Before I went to vacation I think I was 130-135, would occasionally dip under 130 and was trying to get more into the 120s. But now when I step on the scale, it is embarrassing, I am 140-142 pounds. Even after a run I was 139 pounds. I have no idea how I gained it so fast, and none of it has gone away after 3 days even though I am eating well. Running only a bit more than I was before due to race coming up

It's mostly water + glycogen. Your weight will go up and down by up to 10 pounds per day in summer, it's normal.

Never caught it lower than 139-140 in the last 3 days even though I've been weighing when I get up and after runs.

That's because you are well fueled and hydrated. Do you sweat normally (some people don't sweat much)? If so, then you can expect large fluctuations from time to time during hard summer training.

Good news and bad news:

First, you probably did not gain as much as you thought; some of it is bulk mass 'in transit'. Take your weight in a day or so first thing in the morning for consistency.

Second, it is hard to gain 5 pounds in a week and almost impossible to lose that much and race well. Assume that the pounds are all fat -- 3500 calories per pound. Now, run a marathon a day (150- pound guy) and expend 2700 calories - 12 ounces of fat.

Ive been taking it the last 2 days and it has stayed around 140.