Not the drug, but the person.
The heroine should be someone you would like to be or someone they could be best friends with. Since they are one of the two main characters in a romance story, they should be someone that the reader will enjoy. The reader will be spending at least two hours with this person, so they should like them.
(There are of course exceptions to this rule. There are always exceptions.)
We want to live another life. That's why we read fiction. It gives us a chance to fall in love with different people and live lives that we could never have. I will never be a 16th century courtesan, but I can read it and give my brain the sensation that I am. I'd love to fall in love with a handsome billionaire, but my husband would miss me.
That's why I read instead. I get all the thrill, but none of the work.
As such, make your heroine someone that your reader will want to be. Make them funny or sweet. Make their life more exciting. They can have danger and drama without the risks. They should never be boring or rude. We have to like them, even if we don't agree with their decisions.
You can have a strong heroine. She can be brass. She can be ditzy or even a little self-centered, but we have to like her. We need to see something redeemable inside of her. Romance readers want the happily ever after (that's why we read romance and not other genres) so she needs to be capable of not only having that good ending, but deserving it.
Your heroine may not be everyone's cup of tea, but she should always be kind. She needs to be someone you want to spend the next week with (as a writer and a reader.) I'm a shy introvert, so I don't always see myself as the bold, uber-friendly heroines. However, I would LOVE to be their friend. I can get behind a heroine that could be my best friend.
Your heroine can make mistakes. She should make mistakes. If Elizabeth Bennet wasn't prejudiced, we wouldn't have a story. But, the important part is that even though Elizabeth has all the wrong ideas about Darcy, we still like her. We understand why she's sure that Darcy is a prideful snot, and we can see a bit of ourselves behaving that way too.
Keep this in mind while writing your character. The heroine is your reader. No one wants to think of themselves as rude or unkind. We all want to be Cinderella with animals that come rescue us because we are so damn awesome. She shouldn't be perfect, but she should be someone we want to be. Someone we would want to be friends with.
The main point here is that the heroine should be likable. We should want her to succeed. We need to feel that she deserves this chance at happiness.
As you write your heroine, make sure that she's always someone you'd want to have around. Even when she's sad and mopey because she's lost the love or her life, we should want to bring her ice cream. We can want to smack her upside the head, but we should want to do it because we love her and want what's best for her.
Not because she's annoying the crap out of us.
Your heroine should be someone you want to be or someone you want as your best friend.