What is the worst side effect from medication?

That I have personally had?
Just droziness (hope I spelled that right).

Nausea, being light headed and sick to your stomach. (Some hay fever prescription meds did that to me).

I think the worst side effect effect I ever suffered from a med—which, now that I know I’m allergic to, I’ve never taken since—was the rapid paralyzation of my tongue. Fortunately, a trip to the hospital for an antihistamine shot ended it, but for a while I was terrified.

Three way tie between sun sensitivity (especially when I started that medication in the summer), more drowsiness than usual, and loose stools.

I agree with @elbanditoroso (again). Nausea. Some medication can leave me feeling very sick for a couple of hours after I take it.

Other side effects I hate and am not prepared to live with, are fogginess and loss of libido. I took a drug I was prescribed and I couldn’t think straight. I felt vague constantly.

The same drug killed my libido. I have a very healthy sex drive and suddenly the last thing I wanted was sex and if I decided to go ahead and have sex anyway – no orgasm. Couldn’t get there. I went back to the doc and told him what to do with his drug.

When I felt like my skin was burning and an elephant was sitting on my chest. It wouldn’t surprise me if it permanently affected my heart. I took that medicine for ten days. Ten days of poison running into an IV in my arm. Amazing what desperation will cause you to do. I asked my MIL to sleep in bed with me one night (my husband was out of town) because I thought I might die. Only time in my life I seriously thought I might die.

I was on a variant of Ritalin and it made my penis literally shrivel up when I was on it. When I wasn’t on it, everything was fine. Needless to say, I stopped taking that stuff. It really did help with my concentration/productivity, but the price was way too high.

Not that is has happened yet, but the worse side effect would be impotence.

Anaphylactic shock, luckily caught within 60 seconds and fixed. Not an experience I’d want to repeat.

Almost complete loss of cardiovascular stamina (caused by a statin drug). Before my doc prescribed the drug, I easily walked a mile plus at a time, encountering no stamina or shortness of breath effects along the way. Within three days of taking the statin, in addiiton to the extreme muscle pain that is the major side effect, my ability to go anywhere on foot fell off to the point that I was forced to stop every two blocks & rest for 5 – 10 minutes before I felt able to walk another two blocks. Needless to say, I didn’t keep taking that cholesterol lowering drug.

The worst side effect I’ve experienced was vivd nightmares when I tried Inderol.

One of the worst I can imagine would be dizziness and nausea because I hate both of them.

Dizziness and constipation