Trioxsalen is a photosenisitizer. It is obtained from several plants, mainly Psoralea corylifolia. Like other psoralens it causes photosensitization of the skin. It is administered either topically or orally in conjunction with UV-A (the least damaging form of ultraviolet light) for phototherapy treatment of vitiligo and hand eczema. After photoactivation it creates interstrand cross-links in DNA, which can cause programmed cell death unless repaired by cellular mechanisms. In research it can be conjugated to dyes for confocal microscopy and used to visualize sites of DNA damage.

Trimethoprim (TMP) is an antibiotic used mainly in the treatment of bladder infections. Other uses include for middle ear infections and travelers' diarrhea. With sulfamethoxazole or dapsone it may be used for Pneumocystis pneumonia in people with HIV/AIDS. It is taken by mouth. Trimethoprim was first used in 1962.

Trimethobenzamide is an antiemetic used to prevent nausea and vomiting. It is often prescribed for patients with gastroenteritis, medication-induced nausea, and other illnesses. Trimethobenzamide is generally considered the most potent antiemetic that does not have effects on the serotonergic, dopaminergic, or histaminergic systems, so it has a lower likelihood of causing undesired side effects. In the United States, it requires a prescription. Although the specific mechanism through which trimethobenzamide functions is unknown, it is believed to affect the chemoreceptor trigger zone (CTZ) of the medulla oblongata.

Trimethadione is an oxazolidinedione anticonvulsant. It is most commonly used to treat epileptic conditions that are resistant to other treatments. Trimethadione (TMO) has the properties required of probe drugs for the evaluation of hepatic drug-oxidizing capacity in humans in vivo. TMO is demethylated to dimethadione (DMO), its only metabolite, in the liver after oral administration.

Benzhexol, also known as Trihexyphenidyl, is an antiparkinsonian agent of the antimuscarinic class. It has been in clinical usage for decades.

Trichlormethiazide is a diuretic with properties similar to those of hydrochlorothiazide. It is usually administered for the treatment of oedema (including that which is associated with heart failure, hepatic cirrhosis and corticosteroid therapy) and hypertension. In veterinary medicine, trichlormethiazide can be combined with dexamethasone to be used on horses with mild swelling of distal limbs and general bruising. As a diuretic (in particular a thiazide), trichlormethiazide encourages water loss from the body.

Triamcinolone is used to treat a number of different medical conditions, such as eczema, psoriasis, arthritis, allergies, ulcerative colitis, lupus, sympathetic ophthalmia, temporal arteritis, uveitis, ocular inflammation, Urushiol-induced contact dermatitis, aphthous ulcers (usually as triamcinolone acetonide), visualization during vitrectomy and the prevention of asthma attacks. It will not treat an asthma attack once it has already begun. It has also been used off-label for macular degeneration.

Tranylcypromine is a monoamine oxidase inhibitor (MAOI)—it is a nonselective and irreversible inhibitor of the enzyme monoamine oxidase (MAO). It is used as an antidepressant and anxiolytic agent in the clinical treatment of mood and anxiety disorders, respectively.

Trandolapril is an ACE inhibitor used to treat high blood pressure. It acts by competitive inhibition of angiotensin converting enzyme (ACE), a key enzyme in the renin-angiotensin system which plays an important role in regulating blood pressure..

Tolterodine is an antimuscarinic drug that is used for symptomatic treatment of urinary incontinence.

Tolnaftate is a synthetic thiocarbamate used as an anti-fungal agent that may be sold without medical prescription in most jurisdictions. It can be administered as a cream, powder, spray, and liquid aerosol. Tolnaftate is used to treat fungal conditions such as jock itch, athlete's foot and ringworm.

Tolazoline is a non-selective competitive α-adrenergic receptor antagonist. It is a vasodilator that is used to treat spasms of peripheral blood vessels (as in acrocyanosis). It has also been used (in conjunction with Sodium nitroprusside) successfully as an antidote to reverse the severe peripheral vasoconstriction which can occur as a result of overdose with certain 5-HT2A agonist drugs such as 25I-NBOMe, DOB and Bromodragonfly (prolonged severe vasoconstriction can lead to gangrene if untreated).

Tofisopam is an anxiolytic that is marketed in several European countries. Chemically, it is a 2,3-benzodiazepine. Unlike other anxiolytic benzodiazepines (which are generally 1,4- or 1,5-substituted) however, tofisopam does not have anticonvulsant, sedative, skeletal muscle relaxant, motor skill-impairing or amnestic properties. While it may not be an anticonvulsant in and of itself, it has been shown to enhance the anticonvulsant action of classical 1,4-benzodiazepines such as diazepam (but not sodium valproate, carbamazepine, phenobarbital, or phenytoin). Tofisopam is indicated for the treatment of anxiety and alcohol withdrawal.

Tizanidine is a drug that is used as a muscle relaxant. It is a centrally acting α2 adrenergic agonist. It is used to treat the spasms, cramping, and tightness of muscles caused by medical problems such as multiple sclerosis, ALS, spastic diplegia, back pain, or certain other injuries to the spine or central nervous system. It is also prescribed off-label for migraine headaches, as a sleep aid, and as an anticonvulsant. It is also prescribed for some symptoms of fibromyalgia.

Tioconazole is an antifungal medication of the imidazole class used to treat infections caused by a fungus or yeast. It is marketed under the brand names Trosyd and Gyno-Trosyd (Pfizer). Tioconazole ointments serve to treat women's vaginal yeast infections. Tioconazole topical (skin) preparations are also available for ringworm, jock itch, athlete's foot, and tinea versicolor or "sun fungus".

Tinidazole is an antiprotozoal, antibacterial agent. The nitro- group of tinidazole is reduced by cell extracts of Trichomonas. The free nitro- radical generated as a result of this reduction may be responsible for the antiprotozoal activity. Chemically reduced tinidazole was shown to release nitrites and cause damage to purified bacterial DNA in vitro. Additionally, the drug caused DNA base changes in bacterial cells and DNA strand breakage in mammalian cells. The mechanism by which tinidazole exhibits activity against Giardia and Entamoeba species is not known.

Tiludronate is a bisphosphonate used for treatment of Paget's disease of bone (osteitis deformans) in human medicine. In veterinary medicine, tiludronic acid is used to treat navicular disease and bone spavin in horses. Its trade names are Tildren and Equidronate. It is approved for treatment of navicular disease and distal tarsal osteoarthritis in Europe, and was approved for treatment of navicular disease in the United States in 2014. Tiludronate is a non-nitrogenous bisphosphonate that inhibits osteoclasts, the primary cell responsible for the breakdown of bone required for bone remodeling.

Ticarcillin is a carboxypenicillin. It is almost invariably sold and used in combination with clavulanate as Timentin. Because it is a penicillin, it also falls within the larger class of beta-lactam antibiotics. Its main clinical use is as an injectable antibiotic for the treatment of Gram-negative bacteria, particularly Pseudomonas aeruginosa. It is also one of the few antibiotics capable of treating Stenotrophomonas maltophilia infections. It is provided as a white or pale-yellow powder. It is highly soluble in water, but should be dissolved only immediately before use to prevent degradation.

Tiaprofenic acid is a non-steroidal anti-inflammatory drug (NSAID) of the arylpropionic acid (profen) class, used to treat pain, especially arthritic pain Long-term use of tiaprofenic acid is associated with severe cystitis, roughly 100 times more commonly than other NSAIDs. It is contraindicated in patients with cystitis and urinary tract infections.

Thioridazine is a piperidine typical antipsychotic drug belonging to the phenothiazine drug group and was previously widely used in the treatment of schizophrenia and psychosis; the branded product was withdrawn worldwide in 2005 because it caused severe cardiac arrhythmias, however, generic versions are available in the US.

Thiabendazole is a fungicide and parasiticide. Tiabendazole works by inhibition of the mitochondrial, helminth-specific enzyme, fumarate reductase, with possible interaction with endogenous quinone. Tiabendazole serves to block angiogenesis in both frog embryos and human cells. It has also been shown to serve as a vascular disrupting agent to reduce newly established blood vessels. Tiabendazole has been shown to effectively do this in certain cancer cells.

Tetrahydrofolic acid, or tetrahydrofolate, is a folic acid derivative. Tetrahydrofolic acid is a cofactor in many reactions, especially in the metabolism of amino acids and nucleic acids. Tetrahydrofolic acid acts as a donor of a group with one carbon atom. It gets this carbon atom by sequestering formaldehyde produced in other processes.

Terlipressin is an analogue of vasopressin used as a vasoactive drug in the management of low blood pressure. It has been found to be effective when norepinephrine does not help. Indications for use include norepinephrine-resistant septic shock and hepatorenal syndrome. In addition, it is used to treat bleeding esophageal varices. Terlipressin is currently not available in the United States, or Canada, but it is available in New Zealand, Australia, much of Europe, India & UAE.

Terbutaline is a β2 adrenergic receptor agonist, used as a "reliever" inhaler in the management of asthma symptoms and as a tocolytic (anti-contraction medication) to delay preterm labor for up to 48 hours. This time can then be used to administer steroid injections to the mother which help fetal lung maturity and reduce complications of prematurity. It should not be used to prevent preterm labor or delay labor more than 48–72 hours.

Tamsulosin is an α1a adrenergic receptor antagonist used in the symptomatic treatment of benign prostatic hyperplasia (BPH). Tamsulosin is used in the treatment of difficult urination, a common symptom of enlarged prostate. Tamsulosin, and other medications in the class called alpha blockers, work by relaxing bladder neck muscles and muscle fibers in the prostate itself and make it easier to urinate.

Tacrine is a centrally acting anticholinesterase and indirect cholinergic agonist. It is approved for the treatment of Alzheimer's disease.

Sulfisoxazole is a sulfonamide antibacterial with an oxazole substituent. It has antibiotic activity against a wide range of Gram-negative and Gram-positive organisms.

Sulfapyridine is a sulfonamide antibacterial. At one time it was commonly referred to as M&B. Sulfapyridine is no longer prescribed for treatment of infections in humans. However, it may be used to treat linear IgA disease. It is a good antibacterial drug, but its water solubility is very pH dependent. Thus there is a risk of crystallization within the bladder or urethra, which could lead to pain or blockage. As with other sulfonamides, there is a significant risk of agranulocytosis, and this, rather than the development of resistance by bacteria, is the main reason for its decline in use.

Sulfanilamide is a sulfonamide antibacterial. Chemically, it is an organic compound consisting of an aniline derivatized with a sulfonamide group. sulfanilamide remains in use for treatment of vaginal yeast infections.

Sulfamoxole is a sulfonamide antibacterial.