Hyoscine butylbromide is effective in treating crampy abdominal pain.Hyoscine butylbromide is effective in reducing the duration of the first stage of labour, and it is not associated with any obvious adverse outcomes in mother or neonate.It is also used during abdominal or pelvic MRI or CT scans to improve the quality of pictures.

Loperamide is an opioid-receptor agonist and acts on the μ-opioid receptors in the myenteric plexus of the large intestine. It works like morphine, decreasing the activity of the myenteric plexus, which decreases the tone of the longitudinal and circular smooth muscles of the intestinal wall. This increases the time material stays in the intestine, allowing more water to be absorbed from the fecal matter. It also decreases colonic mass movements and suppresses the gastrocolic reflex.

Loperamide's circulation in the bloodstream is limited in two ways. Efflux by P-glycoprotein in the intestinal wall reduces passage of loperamide, and the fraction of drug crossing is then further reduced through first-pass metabolism by the liver.Loperamide metabolizes into an MPTP-like compound, but is unlikely to exert neurotoxicity.

Enkephalins are peptides produced by the body that act on opioid receptors with preference for the δ subtype.^[ Activation of δ receptors inhibits the enzyme adenylyl cyclase, decreasing intracellular levels of the messenger molecule cAMP.

The active metabolite of racecadotril, thiorphan, inhibits enkephalinase enzymes in the intestinal epithelium with an IC₅₀ of 6.1 nM, protecting enkephalins from being broken down by these enzymes. (Racecadotril itself is much less potent at 4500 nM. This reduces diarrhea related hypersecretion in the small intestine without influencing basal secretion. Racecadotril also has no influence on the time substances, bacteria or virus particles stay in the intestine.

Rifaximin interferes with transcription by binding to the β-subunit of bacterial RNA polymerase. This results in the blockage of the translocation step that normally follows the formation of the first phosphodiester bond, which occurs in the transcription process.^[ This in turn results in a reduction of bacteria populations, including gas producing bacteria, which may reduce mucosal inflammation, epithelial dysfunction and visceral hypersensitivity. Rifaximin has broad spectrum antibacterial properties against both gram positive and gram negative anaerobic and aerobic bacteria. As a result of bile acid solubility, its antibacterial action is limited mostly to the small intestine and less so the colon. A resetting of the bacteria composition has also been suggested as a possible mechanism of action for relief of IBS symptoms.Additionally, rifaximin may have a direct anti-inflammatory effect on gut mucosa via modulation of the pregnane X receptor.

Other mechanisms for its therapeutic properties include inhibition of bacterial translocation across the epithelial lining of the intestine, inhibition of adherence of bacteria to the epithelial cells and a reduction in the expression of proinflammatory cytokines.

Tavaborole, when prepared with a 1:1 mixture of ethyl acetate and propylene glycol, has the ability to fully penetrate through the human nail.^{[citation needed]} In studies with cadaver fingernails, a 5% solution of tavaborole penetrated the nail an average of 524.7 mcg/cm² after two weeks of daily use.

Tavaborole is detectable in the blood at a level of 3.54 ng/mL after a single use of 0.2 mL of the 5% solution. Tavaborole has an elimination half-life of 28.5 hours, a maximum concentration of 5.17 ng/mL after two weeks of daily use, and takes 8 days to reach the maximum concentration.

Ritonavir was originally developed as an inhibitor of HIV protease, one of a family of pseudo-C2-symmetric small molecule inhibitors.^{[citation needed]}

Ritonavir is now rarely used for its own antiviral activity but remains widely used as a booster of other protease inhibitors. More specifically, ritonavir is used to inhibit a particular enzyme, in intestines, liver, and elsewhere, that normally metabolizes protease inhibitors, cytochrome P450-3A4 (CYP3A4).The drug binds to and inhibits CYP3A4, so a low dose can be used to enhance other protease inhibitors. This discovery drastically reduced the adverse effects and improved the efficacy of protease inhibitors and HAART. However, because of the general role of CYP3A4 in xenobiotic metabolism, dosing with ritonavir also affects the efficacy of numerous other medications, adding to the challenge of prescribing drugs concurrently.

Lamivudine is an analogue of cytidine. It can inhibit both types (1 and 2) of HIV reverse transcriptase and also the reverse transcriptase of hepatitis B virus. It is phosphorylated to active metabolites that compete for incorporation into viral DNA. They inhibit the HIV reverse transcriptase enzyme competitively and act as a chain terminator of DNA synthesis. The lack of a 3'-OH group in the incorporated nucleoside analogue prevents the formation of the 5' to 3' phosphodiester linkage essential for DNA chain elongation, and therefore, the viral DNA growth is terminated.

Lamivudine is administered by mouth, and it is rapidly absorbed with a bio-availability of over 80%. Some research suggests that lamivudine can cross the blood–brain barrier. Lamivudine is often given in combination with zidovudine, with which it is highly synergistic. Lamivudine treatment has been shown to restore zidovudine sensitivity of previously resistant HIV. Lamivudine showed no evidence of carcinogenicity or mutagenicity in in vivo studies in mice and rats at doses from 10 to 58 times those used in humans.

Aciclo-GTP, the active triphosphate metabolite of aciclovir, is a very potent inhibitor of viral DNA replication. Aciclo-GTP competitively inhibits and inactivates the viral DNA polymerase.Its monophosphate form also incorporates into the viral DNA, resulting in chain termination. It has also been shown that the viral enzymes cannot remove aciclo-GMP from the chain, which results in inhibition of further activity of DNA polymerase. Aciclo-GTP is fairly rapidly metabolized within the cell, possibly by cellular phosphatases.Aciclovir is active against most species in the herpesvirus family.

Imipenem/cilastatin has the ability to kill a wide variety of bacteria. Imipenem is the active antibiotic agent and works by interfering with their ability to form cell walls, so the bacteria break up and die.

Imipenem is rapidly degraded by the renal enzyme dehydropeptidase if administered alone (making it less effective); the metabolites can cause kidney damage. Imipenem is a broad-spectrum betalactam antibiotic used for severe bacterial infections caused by susceptible organisms. Because imipenem is rapidly inactivated by renal dehydropeptidase I, it is given in combination with cilastatin, a DHP-I inhibitor which increases half-life and tissue penetration of imipenem.

Meropenem is bactericidal except against Listeria monocytogenes, where it is bacteriostatic. It inhibits bacterial cell wall synthesis like other β-lactam antibiotics. In contrast to other beta-lactams, it is highly resistant to degradation by β-lactamases or cephalosporinases. In general, resistance arises due to mutations in penicillin-binding proteins, production of metallo-β-lactamases, or resistance to diffusion across the bacterial outer membrane. Unlike imipenem, it is stable to dehydropeptidase-1, so can be given without cilastatin.

In 2016, a synthetic peptide-conjugated PMO (PPMO) was found to inhibit the expression of New Delhi metallo-beta-lactamase, an enzyme that many drug-resistant bacteria use to destroy carbapenems.