Penicillins are a group of drugs. I think it's most accurate to simply use the plural. Over 400,000 hits for the plural.

Examples:

en.wikipedia.org/wiki/Penicillin
Wikipedia
**Penicillins**, like other β-lactam antibiotics, block not only the division of bacteria, including cyanobacteria, but also the division of cyanelles, the photosynthetic

Penicillins - UpToDate
www.uptodate.com/contents/penicillins
UpToDate
Jul 29, 2013 - The classification, spectrum of activity and pharmacology of **one group of beta-lactam antibiotics, the penicillins**, will be reviewed here.

**Penicillins**: Antibacterial Agents: Merck Veterinary Manual
www.merckmanuals.com/vet/pharmacology/...agents/penicillins...
The penicillins are among the earliest classes of antibacterial drugs. Penicillins are further subclassed based on chemical structure (eg, penicillins, ...