A muscle relaxant is a drug which affects skeletal muscle function and decreases the muscle tone. It may be used to alleviate symptoms such as muscle spasm and pain, and hyperreflexia. The term "muscle relaxant" is used to refer to two major therapeutic groups: neuromuscular blockers and spasmolytics. Neuromuscular blockers act by interfering with transmission at the neuromuscular end plate and have no CNS activity. They are often used during surgical procedures and in intensive care and emergency medicine to cause paralysis. Spasmolytics, also known as "centrally-acting" muscle relaxants, are used to alleviate musculoskeletal pain and spasms and to reduce spasticity in a variety of neurological conditions. While both neuromuscular blockers and spasmolytics are often grouped together as muscle relaxants

Phenylpropanolamine (PPA) is a drug of the phenethylamine family used as a decongestant and also as an appetite suppressant. In veterinary medicine, it is used to control urinary incontinence in dogs

In the United States, PPA is no longer sold without a prescription due to a somewhat increased risk of stroke in younger women. It is easily available in Europe and elsewhere in the world.

There are four optical isomers of phenylpropanolamine: d- and l-norephedrine, and d- and l-norpseudoephedrine. D-norpseudoephedrine is also known as cathine, and occurs naturally in the stimulant plant Catha edulis (khat). This isomer is commonly used in European medications described as "phenylpropanolamine", whereas in the United States a racemic mixture of d,l-norephedrine is usual.

Phenylpropanolamine was also used for the illicit synthesis of other stimulant drugs such as phenmetrazine and 4-methylaminorex, and since phenylpropanolamine was withdrawn from use in humans in the early 2000s (although it is still sold for some veterinary applications) it is now much less available, and this in turn has meant that phenmetrazine and 4-methylaminorex have largely disappeared from the illicit market

Colchicine is a highly poisonous natural product and secondary metabolite, originally extracted from plants of the genus Colchicum (Autumn crocus, Colchicum autumnale, also known as the "Meadow saffron"). Originally used to treat rheumatic complaints and especially gout, it was also prescribed for its cathartic and emetic effects. Its present medicinal use is mainly in the treatment of gout; as well, it is being investigated for its potential use as an anti-cancer drug. It can also be used as initial treatment for pericarditis and preventing recurrences of the condition. In neurons, axoplasmic transport is disrupted by colchicine.