Pharmacies deal with medicines, and most medical compounds are small organic molecules like aspirin or ethanol, or larger biological molecules, mostly proteins like insulin or prolactin. So a pharmacist is doing a fair bit of chemistry, in addition to doing biology, medicine, and retail sales.
Pharmacists are not just concerned with how their chemicals affect the body of the patient, but they are also concerned with how the molecules interact with one another, how they affect the body in combination, and how they are delivered. To make a pill that survives the stomach acid, and is delivered into the intestines, can be complicated.
Some drugs must be injected or delivered through the skin in some other way, because they can’t pass through the digesting tract unaltered. Most proteins are in this category. Some are inhaled for the same reason, or because they act faster that way. Knowing how chemistry affects drug delivery is an important part of designing pills, capsules, injectables, and inhalants.
Many drugs act on the body in similar ways, and when used in combination, they can have harmful effects. Pills that make you drowsy are generally not safe to take when drinking alcohol, for example. The alcohol can increase the effects or side-effects of the drug, and the drug can increase the effects of the alcohol.
In other cases, a drug might cause the body to produce an enzyme that breaks down another drug the patient might be taking. Understanding chemistry allows the pharmacist to suggest that the pills be taken at different times, or that another drug be substituted for one of the originals.