Can you take metoprolol tartrate instead of metoprolol er if you have to

They're the same drug, it's just the metoprolol ER is a long-acting version of the other one.

In other words, the dosage and timing would be different. If you were to take the same dose of metoprolol as you do metoprolol ER, you run the risk of getting too much metoprolol released in your system all at once. Since metoprolol tartrate isn't a long-acting drug, you'd need to take lower doses more frequently.

So don't switch except under the guidance of your doctor. That way you'll know you're getting the right amount.