what are the negative effects in believing in superstitions?

Fear

Wasting money, time, resources,

People teasing you....

You drop your Skepticism standards down . . . Once you've done that you are open to all sorts of Supernatural rubbish.

I had a chat one time with a person who told me she used to not be afraid of scary movies . . . Now she cannot watch because of "Paranormal Activity" the movie. Because, according to her, that COULD really happen . . . You know with demons and such.

To which I said . . . "No No No . . . . You avoid certain parts of cities because drug usage, prostitution, and violence COULD happen . . . Supernatural claims have YET to be ever proven TO HAPPEN."

You drop your standards of Skepticism . . . and you might as well be holding a sign that says "Will trade logic for Willful Ignorance and naivety any time, any place, anywhere."

The Dark Ages, The Crusades, The Inquisition, The Holocaust; war, ethnic cleansing (genocides), cannibalism, witch hunts, honor killings, death cults, genital mutilations, blood rituals and human sacrifice; Sharia Law, Jihad, flying planes into buildings; fake thought “crimes” like Apostasy, Heresy and Blasphemy; protesting at veterans’ funerals; shunning medical treatment and vaccines, denial of science, twisted concepts of reality; subjugation of women, pedophile priests, homophobia; ignorance, fear of hell, fear of demons, wishing for the end of the world; and institutionally imposed shame of our thoughts, bodies and species. 

In his book Believing in Magic—The Psychology of Superstition, Professor Stuart A. Vyse warns: “Superstition could lead to diminished quality of life if one spends large sums of money on psychics, fortunetellers, numerologists, or Tarot-card readers, or if one’s superstitious rituals help to maintain problem gambling.” Allowing superstition to rule our lives can have far more serious consequences. Many superstitions serve to allay fears about the future.