Houseplants can be both beautiful and useful. However, quite a few of them can cause severe discomfort, illness and even death if ingested. In some instances, contact should be avoided as well. This isn't to say that you shouldn't continue to enjoy houseplants. However, you should be careful to keep them out of reach of children and pets.
 

  Dumbcane (Dieffenbachia)- Juice causes severe burning of mouth and throat. If swelling causes closure of glottis, death can result in untreated.

Philodendron (Philodendron spp.)- Calcium oxalate crystals in juice cause symptoms similar to those above.

  Century plant (Agave)- Saponins in the root can be fatal.

Azalea (Azalea)- Contains andromedo toxin. May cause severe gastrointestinal irritation and nervous symptoms.

  Oleander (Nerium oleander)- Very poisonous, one leaf can be fatal!

Poinsettia (Euphorbia pulcherrima)- Milky juice can cause irritation of mucous membrane.

  Sago Palm (Cycas)- Causes atexia in hind limbs and hepatitis.

Prayer plant (Maranta)- Ingestion of leaves can poison.

  Chrysanthemum (Crysanthemum)- Contains rotenone and can cause severe dermatitis.

Rubber plant (Ficus)- Severe dermatitis and irritation of mucous membranes can be caused by irritant latex.

  Elephant's ear (Colocasia)- Can cause severe irritation of mouth and tongue.

Caladium (Caladium)- Can cause severe irritation of mouth and tongue.

  Four O'Clock (Mirabilis)- Ingestion of roots or seed can cause gastroenteritis.

Wormwood (Artemisia)- Contains toxic oils which cause brain damage.

  Daffodil (Daffodil)- Alkaloids in bulbs can cause severe gastroenteritis and may be fatal.

African Violet (Saintpaulia spp.)- Leaves and stems are poisonous.

  Sweet Violets (Viola odorata)- Rhizomes and seed cause gastroenteritis.

Rex Begonia (Begonia rex)- Purgative and emetic.

  Hydrangea (Hydrangea)- Ingestion can result in gastroenteritis and bloody diarrhea.

Petunia (Petunia)- Contains poisonous alkaloids.

  English Ivy (Hedera helix)- Berrier and leaves can cause labored respiration, purgation and coma.

Peperomia (Peperomia obtusifolia)- Leaves are poisonous.

  Heliotrope (Heliotrope spp.)- Leaves contain a poisonous alkaloid.

Primrose (Primula)- Can cause sever dermatitis.

  Iris (Iris)- Can cause gastroenteritis.

Eucalyptus (Eucalyptus)- Can cause cyanide pisoning.
 
Please note: This is only a partial list of plants that can be dangerous. Further information can be obtained by visiting your local public library or over the internet. Such as at:
University of Illinois - Toxic Plant Database