In 18 weeks pregnant and I have a fever. Is it safe to take any medications for this?

Yes. It is safe to use tylenol (acetaminophen) just like it says on the bottle. If you are having any pain, especially on your back, you really need to see your doctor as this could be a very serious infection of your kidneys.
Tylenol (acetaminophen) Tylenol (acetaminophen) is safe to take as directed during your pregnancy. But, you need to let your doctor know so you can be checked for bacterial infections (like bladder or kidney infections) or common cold/flu infections, or worse - a uterine infection which can lead to pregnancy loss. I hope you received your flu shot. Talk to your doc the same day you recognize a fever for proper evaluation and treatment.
Acetaminophen. No more than 2, 000 mgs every 24 hours but your best move is to grab the phone and call your doctor and keep him informed.