Does shipping in summer heat without ice packs degrade peptides?

It depends mostly on whether the peptide is a dry lyophilized powder or already mixed into liquid. For sealed lyophilized powder, a few days of summer heat usually does little. Degradation reactions like deamidation, oxidation, and hydrolysis need water to run, so a dry, sealed vial is fairly resistant to short heat exposure. Solid-state stability research (Lai et al., J. Pharm. Sci. 1999) points to moisture and molecular mobility, not heat alone, as the main driver of breakdown in dried peptides. Practically, the bigger summer risk is a cracked vial seal or condensation letting moisture in. Reconstituted (already-dissolved) peptide is the real concern. In solution it degrades much faster, and heat speeds that up. Lab handling guides such as JPT's recommend keeping solutions cold and using dry-ice transport when possible. No ice pack on a sealed powder vial for a short transit is generally low-risk; the same heat on a liquid is not. These are research-use-only materials, not approved for human consumption. For any health question, talk to a licensed provider.