Drugs can be dangerous if not properly prescribed.
Some cases are as simple as not giving a patient a drug that they're allergic to.
Other cases are more complicated and involve questions about the right dosage, duration, and route of administration -- as drugs can be administered in many different ways, such as intravenously, orally, rectally, etc.
If an antibiotic is not administered for the proper duration, at the proper dosage, and via the right route, that is malpractice.
Other ways that drugs can wreak havoc are over-dosages. Pharmacies sometimes mistakenly dispense a drug at 10-times the intended concentration, or nursing may not have read a hand-written order correctly and instead of giving 0.10 mg, 10 mg was given, etc.
Polypharmacy -- or the use/administration of multiple drugs to treat one or more conditions -- is also a problem. Drugs can interact and increase side-effects. One drug's toxic side effect may be tolerable or negligible, but if the patient is given multiple drugs with the same toxic side-effect, that could easily push them over the edge by magnifying the toxic side-effect and causing real harm.
Drugs are complicated. When looking at a medical malpractice case, one needs to have an understanding of pharmacology, pharmacokinetics, pharmacodynamics, mechanism of action, bioavailability, adverse effects, dosing, duration, administration, and combinations/contraindications.