Prozac was so popular after its debut in 1988 that it inspired not one but two best-selling books: Listening to Prozac and Prozac Nation (which was turned into a movie starring Christina Ricci).

It was the first SSRI, or selective-serotonin reuptake inhibitor, approved by the Federal Drug Administration, and 20 later, Prozac is still one of the top five psychiatric drugs prescribed to American adults dealing with depression and anxiety.

While most SSRIs are safe and easily tolerated, they are still psychoactive drugs—so yes, there are Prozac side effects you should know about.

Prozac stay in your system for one to two weeks, says Alison Hermann, M.D., a clinical psychiatrist at Weill Cornell Medicine and New York-Presbyterian Hospital. Which means: “If you’re someone who forgets to take your meds every day, that’s great, says Hermann. “If you’re having side effects, that can make things extra difficult.”

If you decide to try Prozac to manage your depression or anxiety, keep an eye out for these side effects.

1. GI Distress

“By far, the most common Prozac side effects are gastrointestinal, mostly nausea and diarrhea,” says Hermann. Fun fact: “There are actually more cells that have serotonin receptors on them in your gut than there are in the brain, and since Prozac works on the serotonin system, that area can be sensitive to having more serotonin around.” These side effects tend to occur early on in treatment, and can be minimized by starting with a low dose or taking the meds with food.

2. Changes in Arousal

No, we’re not talking about sex—yet. “Some people taking an SSRI for the first time may feel tired or a bit sedated initially, or the opposite, a little revved up or jittery,” says Murrough. “It basically occurs as the neurotransmitters in the brain get used to being exposed to something new.” Again, starting on a low dose can help mitigate this side effect.

3. Suicidal Thinking

Antidepressants and anti-anxiety medications are meant to help you feel better, but in some people, they can actually increase depressive thoughts. “The FDA has issued a black box warning—its strictest warning—that antidepressants can actually increase suicidal thinking in young adults and children,” says Murrough. Yes, it’s rare, but if you’re going to the doctor for depression and the medication makes you feel worse, you should absolutely talk to your doc ASAP.

4. Sexual Side Effects

These side effects tend to present once you’re on a stable dose—and the higher the dose, the more likely you are to experience them. “It can run the gamut, whether you’re a man or a woman: decreased libido, decreased genital sensations, impotence, or difficulty having an orgasm,” says Hermann. “We’re not quite sure why this happens, and anxiety and depression affect sexual functioning and sexual interest, so it can be difficult to figure out if it’s the mental disorder or the medication to blame.” Still, talk to your doc if your sex drive takes a dive.

5. Serotonin Syndrome

This is a rare side effects of medications that work on serotonin, like Prozac, but can occur if you’re on more than one drug that affects serotonin levels—which can overload your system. “With serotonin syndrome, you would have symptoms like fever, agitation, increased reflexes, tremors, sweating, dilated pupils, and diarrhea, and you’d likely have to go to the emergency room,” says Hermann.

6. Changes in Weight or Appetite

A study published in the journal JAMA Psychiatry found that people taking Prozac gained an average of a pound and a half over the course of a year of taking the medication.

7. Low Sodium Levels

“When you’re on any SSRIs, the medication can cause your kidneys to excrete more sodium,” says Hermann. “That can lead to symptoms like headaches, confusion, slurred speech, and general weakness.” She recommends that anyone taking antidepressants or anti-anxiety drugs have at least yearly blood work done to check electrolyte levels.

8. Trouble Sleeping

Like most SSRIs, Prozac can have an effect on sleep quality. “Any time you take a medication that affects the brain, there’s a potential for alterations in arousal,” says Murrough. That can manifest in abnormal dreams, difficulty falling or staying asleep, or even nighttime sweats.

9. Increased or Unusual Bruising or Bleeding

“SSRIs can affect the way platelets aggregate to stop bleeding in some people,” says Murrough. This is almost nonexistent in the general population, but there is a risk of bleeding more easily while taking Prozac. This would be most likely to occur in older patients, says Murrough—but if you notice more bruises than normal or cuts that won’t stop bleeding, definitely bring that to your doctor’s attention.