How long does it take for penicillin to leave your system?

Penicillin will be out of your system after your last dose in about 7.7 hours.
Penicillin has an elimination half-life of approximately 1.4 hours or less.
It takes approximately 5.5 x elimination half-life for a medicine to be cleared from your body which is
5.5 x 1.4 hours = 7.7 hours for penicillin.

However other factors to consider include:
1. How much and how often you have taken the drug.
2. Your metabolic rate – a slower metabolism will increase the time a drug remains in your system.
3. Your age and health – older age and poor health will generally increase the time the drug stays in your system.
4. Body mass – generally the bigger you are the longer a drug will remain in your system.

Elimination of Penicillin is both renal (largely unchanged); and by hepatic inactivation and excretion in bile.