Some are, some are not. Succulents are plants that have parts of them more than normally thickened as a result of water retention. Such plants need to retain water because the are usually to be found in the more arid areas of the world or in areas where soil conditions are unfavorable to other types of plant life, e.g. sea coasts where salt levels in the soil would kill off most other plant life. Water may be retained in the stem, leaves or root. Whilst some succulents may be poisonous to humans, they include some plants that have proven extremely beneficial to man such as the Cacti family (well over 1000 types); the Aloe family (over 500 types), the most famous of which is Aloe Vera; and the Sedum, also known as stonecrops, (more than 600 types) which have been used by man for food, medicinal and ornamental purposes for ages. Purslane is a herb used in french cooking to add a bright lemony flavor, it also can thicken soups. This is the same fleshy plant that grows in driveways, it can also be bought from garden centers(rose moss, portulaca). Agave, while dangerous as a plant because of its spines, is where tequila comes from. There are some varieties that can be fashioned into a crude needle and thread which can be used to suture wounds or repair clothing. Aloe has been used medicinally, pretty much since it was discovered. It is used widely as a topical treatment for inflammation. Internally it has been used as a diuretic, a laxative, to relieve cramps, treat arthritis, fever, uclerative colitis, ulcers, asthma, and even diabetes. Also to thicken soups! Euphorbias can be very toxic. Causing a rash or blistering on the skin and even breathing the vapors can cause respiratory distress. The problem with eupborbias is the extreme diversity they show. From looking like a shrub (poinsettia) to cactus (Pencil cactus) to lumps of nothing ( Euphorbia decaryi ). They do all have the same milky sap, which is the source of the toxins. There are far too many succulents to mention. If you have kids or pets, aloes, begonias, and jades are usually safe. There are many plants that we think are pretty but are actually toxic or down right poisonous.